I think there is a major difference between "regulation" (e.g. CFAA, DMCA, SESTA) and antitrust, which is that monopolies are slow and stable and hard to get rid of in any other way.

Chrome defeated Internet Explorer by leveraging a different monopoly. That meant it had copious funding without needing to make money itself and it got a huge amount of free marketing by being featured on google.com. But just trading one monopoly for another isn't competition. Internet Explorer was a monoculture controlled by a single player, now Chrome is, is that better? Was Microsoft going to add features that allow the platform-independent web to compete with Win32 apps? Is Google going to add features that protect privacy and allow users to keep their data on their own machines? Are they going to care about not making things so complicated that no upstart can create a third party browser ever again? Getting away from that requires actual competition.

Meanwhile the actual antitrust enforcement actions we've seen have largely been stupid garbage (pretty much as expected), but they've also been painful and expensive stupid garbage, which gives the targets an incentive to avoid attracting their attention. And, most importantly, antitrust pretty much by definition only applies to really huge companies. If all they did was show up at the doorstep of any company with more than 50% market share and beat them with clubs until they're weak enough that someone else can take market share from them, that's still doing something useful, and most importantly it isn't doing the most dangerous thing regulation can do, which is the opposite of that -- impact smaller more fragile competitors and upstarts more than huge incumbents, which destroys competition.