I've heard many thinks blame nepotism, lack of life experience, writing room constraints from higher ups, even time given - I do wonder though if it's just the assumption that the audience is thick.
Critical Drinker points to lack of life experience, and I think he's onto something. Like one of the things about 1970s Hollywood, though the pool of writers may have been more male and white than today, lots of them had experience working blue collar jobs, or as military veterans, or just doing things outside the creative industries. And that showed in the stories they wrote.
These days it's mostly nepo babies and/or people who've been through the necessary degrees at elite colleges.
I feel the best of old Who comes from writers like Robert Holmes or Malcolm Hulke who had experience in the real world, and that comes through in how they wrote characters. There are lots of writers now who have done all the courses and know all the tropes but don't have that real life background. It shows.