Going with Masterclass is probably the path of least resistance for him to create high-quality video content while continuing to spend most time on research and other educational activities.
Another possible factor: Many good mathematicians are quite perfectionistic and would not want to create homemade-quality videos if they can avoid it.
I think you overestimate how commercially viable actual teaching is as opposed to infotainment. Either way, infotainment allows him to earn money and talk about a subject he loves. Good for him and people love infotainment, it tickles their brains just enough but not too much to require actual thinking.
An example: https://www.theguardian.com/world/2015/jan/16/south-korea-te...
“Teaching pays: Cha said he earned $8m last year. “I’m madly in love with maths,” he said, looking the height of trendiness in his crimson shirt and trousers and tweed jacket, in his office in Gangnam – a wealthy part of Seoul famous for its conspicuous consumption and featured in the song Gangnam Style.”
There is a reason the PISA results of some countries stay consistently near the top: https://factsmaps.com/pisa-2018-worldwide-ranking-average-sc...
That's possible. It's also possible that Terrance Tao is correct in his judgement that using Masterclass was worth it for him. Personally I'd give him the benefit of the doubt that he's not lying.
A lot of these youtubers who put out fantastic videos do essentially all the same things as the Masterclass production crew, but just on a smaller scale. Countless hours are poured into producing these youtube videos that are maybe just as short as 10 minutes.
And even if Terrance Tao wants to make some infotainment, what's wrong with that?
Whether or not something's infotainment and whether or not it could be done more cheaply are orthogonal. There's no reason for him to make this on Youtube, any more than there's a reason for plumbers to do plumbing for nothing in their spare time.