Probably because the similar suggestions many sites have aren't very good. The website doesn't know the contents of the manga beyond the tags it's labeled with, so you end up with the suggestions for "similar" manga that aren't even close to being similar. I hate this on many of the large manhwa group sites (looking at you Asura, et al).
The one way to make sure you end up with at least somewhat accurate similar suggestions is to do something like how myanimelist does it, where users have to manually link what they consider to be similar series, but that's a lot of work and subject to user trolling.