I can make an attempt at this, but I must clarify that I cannot answer with a universal truth here. There's going to be different motivations for this so I'll share my experience.

From my experience, when there are sufficient “resources" at a company and within teams, it doesn't make a ton of sense for engineering leaders to be hands on (as in, actively writing code). While it is still beneficial for managers to be technical (i.e. can relate to their engineers, understand their technical strengths and weaknesses more easily, help guide architecture, etc…) it's not scalable for engineering leadership to be hands on writing code. Leaders can be significantly more impactful by elevating and empowering their engineering teams.

Okay, so why do some places expect this? Probably because they're trying to build technical excellence as they scale up their teams as well. Generally, I observe this at smaller companies or on smaller teams (even at big companies) when things are sort of in an early growth phase.

My personal experience with this was at a startup that I worked at for 8 years (hundreds of employees when I was gone). I actively wrote software on small teams while managing them as well. I was able to influence technical direction and help build up my engineers along the way. Some of my the other larger teams had managers that were not hands on at all (and that's not throwing shade). Their teams were big enough that they *needed* to dedicate full time attention to managing and leading the engineers.

Their areas were also better understood. For additional context, my team actually prototyped or worked on the early versions of the products the other teams would scale out.

So there could be many reasons why, but my best guess would be to try and establish some focus on technical direction as the engineering team is scaled up.

Hope this perspective helps!