The answer is Dealers. Their dealers would sue them if they lost all the revenue they get from updating customers vehicles. On the flipside, it would save the manufacturers a ton of money because they wouldn't be paying dealers to perform these updates when they're under warranty. This also explains why Tesla has to do OTA updates... no dealers.

Most Tesla customers live nearish to a Tesla repair center. So it's not like Tesla is forced to do OTA, but it sure is convenient to have the frequent updates that OTA enables.

Probably because they would have to support tens of models and tens of millions of vehicles. Big car companies contract their head units out per body style, so one company may design the 2008-2012 head units and another 2013-2016. They likely have wildly different sensors, firmware update formats, etc.

Bigger car companies prefer stability over features and don't design everything in-house like Tesla. Their shareholders also expect them to turn profits, which leads to risk adversion and slower product cycles.

The head units can also be vastly different by region - which doesn't really make much sense to me. Sure in US you have satellite radio which is unheard of elsewhere, but that's about it. I have a 2010 Prius and the US model has a DVD for navigation, where as the EU model has a HDD and needs to go to a dealer for €€€ updates.