Why Google Analytics 4 has fewer conversions than Facebook and TikTok?

It's likely view-through conversions because your customer sees ads on several platforms but buys through the click from one, which is attributed in GA.

Repeat purchases that are not done from ad clicks but customers are shown the ads is also a possibility; it's incredible how much impact it can have. Able CDP recently ran an experiment with one of our clients and, when we stopped sending recurring sales to Facebook, the Ads Manager figures matched GA data quite closely, whereas before it had been all over the place. Whether it's desirable or not is an open question; you might argue that you may not get repeat business if your brand isn't in front of the customer regularly.

In Facebook, many clicks are made to the page rather than to the ad link. If you have direct links without UTM parameters on your FB page, then these visits (ad -> page -> website) will be attributed either as direct or Organic Social in Google Analytics 4, but Facebook would obviously know who they are and attribute them to the ads.

It's possible that you've encountered a discrepancy between attribution dates: Google Analytics reports conversions by the date of conversion (which is great for a high-level LTV picture, as you know where your today's revenue comes from), whereas Facebook and other ad platforms report them by the date of acquisition (so you can get a good idea of the RoAS).