Why are Greek letters so widely used as symbols in physics literature when English letters would have served the purpose equally well?

Yes, it is very much a convention. It stems from the rebirth of knowledge in the early renaissance when Italy and the West rediscovered the science found in old Greek works. The rebirth of scientific knowledge more or less placed Greek works on a pedestal. Originally, when science was conducted by writing letters to other scientists, it was not any harder to add Greek letters than it was to use any other pen stroke. And for printing this just required a few more pieces of type, really no harder than normal printing. You are now finding limitations because the computer keyboard was designed for natural language writing, not for scientific notation. If enough people used Greek letters as a convention for their work, you'd probably see these letters installed on all keyboards. But they don't.

Because the Greek alphabet is more beautiful than the English alphabet. :-) The question would have been equally the same if you substituted "English" with any other language in the world. There is no need in such a question to introduce another language besides Greek because the premise of the question remains valid anyway. This way your question is biased and lies on the wrong assumption that English is perhaps superior. It also infers that English would have served the purpose equally well, which is a questionable statement (I personally disagree with for several reasons). The other answers do a good job in explaining the origins of the Greek influence on physics notation.

The premise is not quite true. There are too many constants and units for the roman alphabet to fill all of them. Hence, greek letters are used as they are somewhat familiar to roman users (a or [math]\alpha[/math]) but different enough to use as a extension of the alphabet. Also, it's probably to honour the ancient greek mathematicians who made the foundations of modern maths.

You run out of letters.  For example E is already taken as energy, and L is already taken as angular momentum.  At this point you have the choice of either using something totally unrelated, but even "q" is taken for charge, or you use greek.  Lambda is used for length scales because it reminds you of L, but L is already taken. This is speculation, but I suspect this also has to do with printing.  If you go to a printer in 18th/19th century Europe and ask them to print something using Chinese characters or letters that you invented, they couldn't.  However, because printers had to print things like Greek versions of classical literature and the Bible, if you wanted them to insert Greek letters in English text, they could do this. Something else to point out is that English wasn't the language of science until after World War II.  Before the war, the language of science was German, and if you go back even further, it was Latin.

The point of the Greek alphabet was precisely that it was not the English alphabet. It was for things that should not be confused with letters. So that in a manuscript you were not confused between an a as a physical constant, and an a as the first letter of a word. If it hadn't been such a pain, they would probably have written them in red or some other way of distinguished values from words. But Greek was familiar enough to use its letters instead.

German black letter were once used, then Claredons (bold), then greekish letters.  In part one thinks of what might be in a printer's type-set, especially a university one. I use greek letters as modifiers in place of dotting etc, so eg http://www.os2fan2.com/tmp/physics.pdf gives a list of my usage.

I'm in agreement with what others have written with a couple of comments: Greek is a "natural" choice because the Greeks invented a lot of basic mathematics and geometry.  Ancient Greece is often considered to the origin of the European intellectual tradition, not just in mathematics.  It's partly a kind of reverence thing and partly a simple issue of availability: a student attending universities like Bologna, Oxford or Heidelberg in say 1650 would have almost certainly learned both Latin and Greek and drawn symbols from both these alphabets for use in mathematics. After a few centuries you find you have a tradition. The Greek letters that are commonly used are ones that either don't look like a Latin/English character, or, if they do, a conventional way of writing them has been developed that looks clearly different.  Letters like lower case alpha and nu are written in a particular way in physics so they are distinctively different to the Latin "a" and "v" so that both could be used in an equation.  In Greek and English newspapers they will usually look the same.   The Greek letter Omicron which looks like the English O in either upper or lower case - and also looks like a zero - is basically avoided since it adds nothing to the set of symbols except perhaps confusion. While each letter may have multiple conventional meanings across science and engineering we find that it is generally clear what you are talking about within a discipline or area.  Standard symbol meanings morph as you move disciplines to cover what is required and to avoid collisions.