Why are DSLR lenses measured at F stops instead of T stops?

Because in the cinema, it is common to change the lenses within a session while maintaining an identical exposure. This is rarely important in still photography (and even less so with the flexibility of digital).

You could say But t-stops are more accurate, allowing me to be more precise! – and that's basically correct, but the main thing is that accuracy is overrated in photography. In general, if the difference is below one stop, it is easy to compensate without too many consequences, and if it is below a third of a stop (a typical variation in t stops), it is barely noticeable, but if you saw that change In a movie, I could jump on you.

Presumably, the transition from film to digital cinema. further makes this less important as it is easier to fix everything in the post. (That's the answer to everything, right?) But that's not an area that I really know something about, so I'm just speculating.

Of course, streaming isn't everything. Aperture also influences depth of field, though I'd say this doesn't matter too much in terms of labeling either, basically for the same reason of precision: the distance of the subject (and the focus), the display size of the print and all the factors that affect the depth of field mean that just knowing the f number gives you only an idea general results. (And that's not even considering that the numbers are often rounded.)