Because of the large and rapidly-increasing number of sites that just refuse to work if you are running an ad blocker, unless you manually disable it first.
Can you link any site that doesn't work? It is easy enough to try and get past anti-adblock with uBlock Origin.
Try uBlock Origin. Don't use stuff like 'Adblock Plus/Pro'.
I think I can count on my fingers the number of instances of this that I've seen in my many years of using an ad blocker, and I don't even remember the last time it happened. To claim that some tiny percentage of websites doing this makes ad blockers "useless" is completely ridiculous.
they are LARGELY useful...
they do what they say on the tin, at least unless you visit some super agressive site which uses anti-adblock, but, then again you can conf up an anti-anti-adblock ;)