I think painted posters are less to do with cost (as compared to a marketing budget, a painted poster is likely pretty cheap) and more to do with how posters became simpler and more immediate.
So much detail is lost on a streaming thumbnail, on phones, etc.
A digital poster with a large face/image or a collage of recognizable faces, done in the most cleaned-up way possible for peak recognition, is far more desirable for a major film studio.
Film posters became cheap looking but what we see a majority of the time is much more easily digestible for a wandering eye.