Because they don't refuse access. They kindly ask you to disable, but don't require it, and the users of an ad blocker know damn well they're not going to voluntarily disable it if it's possible to browse the site at all with it on, so rather than rote click "continue" 90 times every two hours, let the blocker block the nagging popup as well.
If these sites really don't want you there with an ad blocker, they can actually block you. I imagine they realize there is value in getting more eyeballs even without ads, whether that be that users might still recommend and create links to your content that other people without a blocker then follow, or you actually sell something and some of those people might buy it. Whatever it is, by revealed preference, those site owners seem to be indicating it's worth it to allow people to browse with an ad blocker.
By default we try to work around sites which require that the user disable their content blocker, as this is not acceptable since content blockers are used for more than to just block advertisement. In uBO, trackers and malware sites are also blocked, and thus it's not acceptable that sites require to wholly disable uBO.
By default, we do not try to work around sites which make a polite demand that the users disable their blockers without preventing access, this way the final decision is up to the users.
Nope; uBlock Origin will work around that as well. Skimming recent issues, in https://github.com/uBlockOrigin/uAssets/issues/15338 atozmath.com detects that people are running an ad blocker [1], explains what they're doing, and refuses access. The discussion is how to make the site think ads aren't blocked, though they haven't succeeded yet. In https://github.com/uBlockOrigin/uAssets/issues/13801 there's a simpler one with https://www.magicgameworld.com where it refuses access but doesn't provide reasoning, and that one was fixed.
If these sites really don't want you there with an ad blocker, they can actually block you. I imagine they realize there is value in getting more eyeballs even without ads, whether that be that users might still recommend and create links to your content that other people without a blocker then follow, or you actually sell something and some of those people might buy it. Whatever it is, by revealed preference, those site owners seem to be indicating it's worth it to allow people to browse with an ad blocker.