It's because the 737 MAX went through significant changes and lobbies the FAA to avoid recertification.

Essentially we have a record which planes have gone through a rigorous certification process in their current configuration and which haven't because looking back it's plain as day.

The design of most planes has been checked properly because the FAA and Boeing have usually done their job properly. In the case of this change to the 737 they haven't.

I'd still recommend requesting a flight on another companies airplane when possible and never accepting a ticket on a 737 max even if it's allowed back in the air.

But there's no need to cause a mass grounding of safe aircraft that don't have any problems. That would be incredibly wasteful and more importantly bring older aircraft into service as an alternative. Older aircraft which would be less safe than the ones on the ground.

Because a large majority of commercial jets in the US are made by Boeing. And grounding more than half of the planes in the US would be terrible for the economy.

Also, different models have different designs, were designed at different times. Many of the planes are 'tried-and-true'.