Anticheats started moving into ring0 and became rootkits in early 2000s with nProtect GameGuard, HWID checks started in mid-2000s with myAC (or even earlier? I think myAC first appeared during CS 1.5).

Probably around the time individual games started to have the term "billion dollars" associated with them in some way.

I wonder if it has anything to do with the rise of e-sports and the monetary aspect of "professional" multi-player games?

As someone who has played a decent of competitive games in the past, it is less about them being extremely serious, and more about how much people cheating ruins the fun for everyone else.

For example, a soccer Sunday league might not be that serious, but you still wouldn't want someone frequently running on the pitch with a golf club and whacking the ball into your goal with no consequences.

When tryhards flocked to ranked games; more broadly in the world with no opportunity for the young people e-sports emerged and they saw it as one.