From a business perspective, you need to assess the impact of the regulation on your profitabiity and then consider if investing business funds elsewhere would lead to greater profitability.

WhatsApp have a single product and have market dominance due to first mover advantage (e.g. everyone is on WhatsApp, so everyone uses WhatsApp). Due to the nature of the business pulling out doesn't make sense unless they only have a limited development team and having them work on UK legal requirements prevents them working on EU requirements, however they are largely similar...

Many 'BigTech' products were developed by small teams, the biggest barrier for entering the market isn't technology but user adoption (KBin, Mastodon, PeerTube & Lemmy demonstrate this, all were developed by 1-2 people in their spare time).

So a 'BigTech' company exiting would be giving up the market in that country and any profit and creating an opportunity for a new small company to grow and eventually compete with them. For example is Facebook pulled out, I'm guessing people would switch to NextDoor.

The US Technology sector is filled with Libretarians who get upset at the idea of regulation. I'm not sure Shareholders/Venture Capitalists would react well to them being irrational.