We have in the past! And we'd love to in the future. However, Grant doesn't need sponsors any longer since his Patreon is healthy, which is awesome.

They actually have sponsored 3b1b in the past.

3Blue1Brown went sponsor-free recently:

https://www.patreon.com/posts/19586800

Discussed here:

https://news.ycombinator.com/item?id=17639322