Why can't multiple homes share one satellite dish?

They can, and they do. I live in such a building. We have Dish network in my building. And the thing is, the people who install Dish network are independent contractors. So, there's an economic incentive for the contractor to tell the tenant that they need their own dish; if the lie succeeds, the installation fee can be collected. This apparently is normal practice. My landlord insisted on being present when the Dish guy came. Sure enough, the installer tried the new dish song and dance, and my landlord told him that he knew otherwise. The installer backed down eventually. I asked my landlord afterward, and he says it usually comes to a standoff, where the tenant & landlord have to threaten to cancel service before it ever begins. But it can be done. My guess is that in the houses with multiple dishes, you either don't have the institutional knowledge or the commitment to service that my landlord displayed.

One satellite can feed multiple receiver. But there are some pretty big gotchas when you start talking about having one satellite feed an entire building. First is that DirecTV and Dish use multiple LNBs to receive feeds from multiple satellites. This often results in 2, 3 or 4 cables needing to be run to each receiver from the satellite. As you might imagine, the costs of running 3 or 4 coax cables from a satellite on the roof or other side of the building to the furthest apartments can be fairly substantial. While some buildings are wired for a single coax feed, it's pretty rare that any but the most modern building have all of the wiring necessary for today's HD signals. The building owner is left with a choice: spend a whole lot on re-wiring everyone's apartments - some who may not use satellite television at all - or let those who want satellite TV service get their own and have a shorter cable run from their own satellite dishes. Second, in order for the signal from a satellite dish to be distributed across long wiring runs and many receivers that get the signals simultaneously, fairly substantial microswitches and amplifiers would need to be installed to distribute the signal throughout the building. Again, this is an expense that a building owner is unlikely to incur if not all of the units are going to request or require satellite TV. Of course, in hotels, hospitals or other locations where it is expected that every room have a feed, a single dish *does* provide a feed to the entire building. But that only happens when such services can both be expected to be widely used and planned as part of the construction or renovation of the building.

They can, at least in the case of DirecTV. (The phrase you want to search for is DirecTV MDU.) The question here is one of who maintains the system. In a condominium, where there are owners for each individual unit, this is easy since it can be done at the behest of the condo association. For apartment complexes, what incentive does the owner have to pay for the installation and maintenance? Even having a "master" satellite system doesn't prevent tenants from installing their own dishes, so the landlord doesn't gain much (an advertising bullet) and DirecTV can't guarantee that people will use their system since some customers might want DISH or cable or nothing at all.

Er, one satellite can feed multiple receivers.

The problem is that the dish receives signals that are polarized one way or another (a way to get more stations on the same frequency band). There's actually a switch in the LBN down converter in the dish to select which polarization matches which channel you are receiving. So if you hook multiple receivers to a standard dish, they can all only receive channels that correspond to the same polarity. New HD dishes have 'multiswitches' that can switch both polarities to up to four receivers. And you can get external multiswitches that can extend the two polarities from the dish to multiple destinations. Looking at Direct TV's MDU page mentioned previously: http://www.directv.com/DTVAPP/global/contentPage.jsp?assetId=3280005 they offer the multiswitch solution, and an IP solution, where they convert all the signals to a Gigabit Ethernet signal which can go to any number of destinations.

I have lived in such a building, as well. The landlord paid for some basic level of television service. I think it was possible to upgrade, but we never had any inclination to investigate. I've also lived in a condo building that shared one cable connection, paid for out of the assessment. It seemingly genuinely was one connection, in that the wiring for one unit depended on the others be connected (at some point we discovered we had a room with a cable jack that wasn't connected to the chain), which puzzled the heck out of the bloke from the cable company.

In many buildings, the landlord doesn't want to be bothered with maintaining the dish for a group of tenants, but is happy for each tenant who wants a dish to get their own and maintain it themselves.

In addition to the above reasons people often want their own independent dishes so that they can pirate or receive foreign Free To Air Transmissions. There is quite a bit of http://www.sat-sales.com/fta_channels.php if you point your dish at the right satellite. FTA satellite is pretty popular with some immigrant groups here.