Not who you replied to, but one view might be something along the lines of "We can't break up these American tech monopolies because at least they're American. If we break up these companies European or - worse! - Chinese or Russian companies will use their outsized scale to simply replace the American companies we kneecapped and now we're worse off".

That is, some people might see it as a lesser of two evils situation where you're choosing the domestic monopolist rather than the foreign one. I think there are certain worldviews where this is worth enough to sacrifice privacy, competition, etc.