One word:

Balance.

Caps are in place in every MMO to date -- some are higher than others, some MMOs take 10s of millions of exp per next level, thus those caps are higher.

I'm more concerned about the 10 second chain 5+ time limit to be honest.
If you've played FFXI before and ever done Merit parties back in the day you'll know this 10 seconds isn't as short as it may seem initially. Basically chain 4 is well enough because if there's enough mobs you'll chain endlessly, usually chain 5+ is when your party has a momentum going and you just begin plowing through enemies.