The AR15/M16 family has an astonishingly wide range of add ons. It's a popular one to own and is quite modular, so there's just a huge market.
If you narrow your search down to quality accessories there's less to contend with (much of what can be bolted on is crap), and even less so if you try and stick to functional items on a given theme (do you really need an ACOG, light, bipod, AND VFG on one rifle?).
I knew there were a lot of accessories out there. I never knew it went beyond light and laser systems, sight systems and suppressors. Well maybe a bayonet. I just thought it was lots of companies making the same things.