You're phrasing this as if Epic is going around making all these poor developers do their bidding - the developer is agreeing to list their game as an exclusive.

Be mad at the dev for making the choice to host only on Epic Store if you're upset about it - they're aware of the consequences and could have easily gone with Steam instead.

It's often a decision of the publishers, not the devs.

The monopoly store doesn't need exclusives, they're already the monopoly. And back when Steam was first starting out, there weren't competing storefronts that they could demand developers withhold their games from.