What's the motivation for this? Regulatory, I get that. But why use direct mechanical linkage in an era where fly-by-wire would probably be faster/safer/etc?
Cost. Not just cost of manufacture/design, but billing it as cheap because you won't have to retrain all your old 737 pilots.
Not training the pilots properly was a feature (cheaper for the airlines), and Boeing deliberately wanted to avoid saying they would actually need the training because that was their whole selling point and the reason for all these weird decisions.
Because the plane would have certainly then required expensive/time consuming re-certification and expensive/time consuming (for the airlines) retraining of pilots, which Boeing was trying to avoid with the max. Keep in mind, the 737 originally flew in the late 60s. It's obviously now a very different aircraft, but if they were going to go fly-by-wire and require re-certification, they'd have designed a whole new aircraft (something a lot of people would have preferred even before the crashes).
The 373 max was a comparative rush job. When Airbus released the A320neo (the A320 first flew in the late 1980s so already was fly-by-wire), Boeing needed to get a comparable plane out ASAP or else cede several thousands of plane sales to Airbus. There wasn't the time to design a whole new plane, so they pushed the design of the 1960s era 373 as far as they could go. The details of the "risky" changes to accommodate the larger engines (that significantly changed the planes aerodynamic profile) and attempts to compensate for that are already documented elsewhere, but adding fly-by-wire would have only made things more complicated.
I'm not a pilot, but from what I have read many pilots actually prefer mechanical linkage to the control surfaces. Sullenberger even indirectly blamed fly-by-wire systems (in this case the lack of a link between pilot and copilot controls) for the crash of Air France 447: https://www.cbsnews.com/news/air-france-flight-447s-lessons-...
One of the most popular features of Boeing aircraft is that there is a mechanical linkage between the yokes of the pilot and copilot.
Boeing's modern planes (not the legacy 737) have fly by wire where there is still a mechanical connection between the yokes. Thus you have the nice shared feel, but you have the benefits of fly-by-wire.
There have been accidents where fly-by-wire has been part of the problem but there have also been failures of the old mechanical linkage systems. Airliners have had active systems to cancel out unstable modes for a long time (e.g. to suppress "Dutch Roll" on the old 727)
The A320 has particularly been plagued by extreme "human error" situations where people crashed the plane after seemingly trying to crash it. For instance the first passenger flight involved a stunt that resulted in a crash. later on New Zealand regulators who were investigating fly-by-wire glitches tried to provoke the fly-by-wire system into failing when they were approaching a runway and they wound up dead.