For me the reason is simple - Transformers are just so FUN!
More specific reasons include:
  • 2 (or more) toys in 1

  • Marvelling at and having with with how they can change between modes

  • The anime-mecha flavour of their designs just looks awesome. Even though some lines have deviated from this (e.g. movieverse, Animated etc.), they're all still inherently spawned from this central point -- a common "DNA" that runs through the entire franchise

  • With post G2 toys, also experiencing how fun Transformers can be not as convertible toys, but as articulated action figures

  • Enjoying the good gimmicks which complement the toy well without compromising it; this also includes gestalts (which is a clever way to make us complete entire teams/sets)

  • Enjoying the mythos/canon. These aren't just toys, they're characters

Transformers are just so much darn fun to play with! One reason why I don't keep Transformers on my work desk is because I know that I'd never get any work done because I wouldn't be able to stop playing with them.