Why have plants not changed or evolved much compared to other life forms?

There are several mistakes and misconceptions in the question and its comment, so I think it's worth taking care of them before answering. The most important of these is the misconception of macroevolution as progress. It simply doesn't work that way, because at the core of it, macroevolution is simply a game of tradeoffs. You cite legs, eyes, and brain (plants have organs, so I took that out). For all the benefits these bring, they also bring a great cost: energy expenditure. Brains and sensory organs require a lot of energy to develop and power; legs require a constant source of food to work. So once you look at the broad picture, it becomes clear that there is no such thing as progress, unless you look at evolution as a directed process leading to a specified outcome - which it is decidedly not. There is no "pinnacle prime" - there is only "adequate for survival in the current environment". Once you understand that, I want you to look at a complete tree of life and note how many branches are multicellular. There's only a handful in the eukaryotes among the tremendous diversity of unicellular eukaryotic, bacterial, and archaeal clades. So the argument that multicellularity and its associated coolness (organs and all that) are somehow a pinnacle that all organisms must aim for is plain wrong. With these two points digested, I hope you can see why your question is very problematic. That said, I fully understand what you mean, so time to answer about plants specifically. Their "basic definition" has not remained the same in their evolution. That picture shows how algal evolution alone proceeded. Landpflanzen means "land plants". That's right: all plants on land today are actually derivatives of algae. Just as all animals are derivatives of colonial choanoflagellates. There is no objective difference between these two cases that makes animals somehow superior in "evolutionary ability". Now let's look at land plants. See the relevant sections in http://bioteaching.wordpress.com/2011/06/25/terrestrialisation/ and observe how many changes they've been through in time. You can't say that the basic body plan of a moss is the same as that of a conifer. And http://bioteaching.wordpress.com/2011/06/29/the-origin-of-modern-biodiversity-coevolution-of-flowers-and-insects/ for the explosive rise of flowering plants with insects. And that opens up another can of worms: the coevolution between flowers and insects. You have flowering plants that have nothing to do with insects - grasses, who've evolved to be pollinated by wind. Then  you have the huge diversity of flowering plants, who've evolved a mindblowing number of ways to attract insects and pollinators. UV guides on the flowers (coevolution with insect vision!), volatiles that appeal to single sexes of single species because they mimic sex pheromones, orchids that have evolved to resemble the females of the male insect pollinators. And if that's not enough, remember that plants have to stay put their entire lives, through all weather changes. Unlike animals, they can't run away and shelter themselves. What this has led to is really awesome physiology. Shedding leaves in winter. Regulation of flowering time by counting hours of daytime or nighttime. All the tropisms that allow therm to grow optimally in any conditions, from seedlings that sprout only after fire to all of plants' tendencies to grow towards light (which is regulated by hormones). They have a ton of defensive systems: grasses concentrate silica balls to destroy horse teeth; plants react in real-time and even communicate with each other to deter herbivores by producing toxins; heck, even thorns are enough to deter most. They're not just sitting ducks, as you imply. They're helpless against us humans and our axes, but so is every animal against our guns.

Plants operate in a very low energy domain. The energy they get from sunlight and absorbed nutrients is enough to grow, but not enough for real mobility. There is little evolutionary advantage in developing animal-like senses if you cannot move to defend yourself. Plants' inability to move doesn't significantly limit the cleverness with which they survive, though. They produce toxins that protect them from predators or inhibit other plants. They reach deep underground to obtain water, trap rain water or even, in the case of air plants, water and nutrients that run off host trees. Some smell like carrion to attract flies for pollination. Their seeds travel on silken parachutes, attach to passing animals, explode through the air like tiny grenades, or travel in the guts of birds or animals. Some seeds remain viable for centuries. Some plants are clones, but they nevertheless evolve. Plants reproduce by suckering, by creating bulbils, via seeds, by underground root extension; sexually or asexually. They strike up collaborative agreements with fungi, bacteria, and algae. Flowering plants flaunt shameless sexuality to attract pollinators. Not all plants depend on the sun -- some parasitize other plants. Others have evolved to specialize in the blue-green wavelengths that make it to the bottom of forest floors. Others eat insects. Many make their own fertilizer, by working together with bacteria that have mastered this trick. So plants are in continuous, active evolution, just like animals. But their basic energy limitations make them look to us like they are relatively static. Until, that is, you look more closely.

A very simple dumbed down answer would be that in comparison to the time each species on earth have lived or lived in, plants have been around for an eternity longer and they have long since evolved into its pinnacle prime. What you see today may be a much advanced form of what it was before. It adapts well, recovers well, has its own form of self defense and is capable of long term survivability.