Probably because they are illegal
Not that it really matters, obviously. But technically DOT only allows 55w or 1500 lumens in certain bulbs. It seems like this must have either been relaxed, or is simply never enforced, as some new cars from the factory have ignorantly bright low beam lights that are downright blinding to oncoming traffic.
Obviously, like many things, the often used "off road use only" loophole is thrown around.
I had a difficult time finding 65w H4 bulbs in the European style lamps for my older Transporters (that in US spec trim got sealed beams). But now with the internet, there are dozens of them available on eBay from all sorts of [usually Chinese] vendors.
But finding them at brick 'n mortar stores may be a bit more difficult. It would be like buying emissions equipment delete parts at Autozone.
Of course, Autozone and others will gladly sell these stadium lighting LED kits that the rednecks grace the front of their brodozers with, which is really annoying. And I am sure tickets could be issued for them, but like the stupid (and illegal) straight pipes on Harleys, it is all too often overlooked.