I hate signing work. Here's a good rationale, from an article on Tibetan thangka painting on PingMag:

Lastly, could you tell me a bit about some of the older thangka that you have in the shop? Who painted them?

We don’t know for sure because thangka painters don’t usually put a date or their name on them. It happens sometimes with new thangka: if you want the date and name, my husband can write it for you, but normally he doesn’t. Thangka are Buddhist images meant for prayer, so it doesn’t make sense to have the artist’s name on it: he’s not the person you are praying to.