Presumably because an adblocker, like virus detection, consumes potentially non-trivial CPU, and may introduce race conditions of extensions applying deltas to a DOM which results in rendering and behavior bugs.

adblockers are just filters and aren't scanning your files like an antivirus, little cpu used I imagine.

uBlock origin's lede is "Finally, an efficient blocker. Easy on CPU and memory." Ad blocking, pre-ublock, could take noticeable resources.

The different apps do the same thing and end up competing for resources.