Because most of the time you don't get hired at FB for a specific team, you might end up working on PHP sure, but the interview process for SWE is general and they test you on algorithms regardless.
Your post seems to indicate that "working on PHP" and "algorithms" are disjoint. Why? If you're writing code with many users, then algorithmic knowledge helps, regardless of language.
Although I will say I think dynamic programming is not a good interview topic. I think it's basically proxy for "got a CS degree from certain schools". But algorithms in general are valid to ask about.
I would almost say you need to understand algorithms better while programming in PHP because the language obscures certain things. Ditto for programming in JS -- for example to encode certain algorithms in JS you will run into the fact that it has only has floats, no ints.
People used to think you couldn't write algorithms in JS either. Now there are LALR(1) parser generators in JS, etc.
