(Yes, this is a few days late, but I was offline for a few days, and boy does it take a long time to catch up on this thread, and I haven't seen an answer to this one ..)

The same reason you buy HP and Dell (or RHEL and Oracle Linux). . Even though it may require a bit more effort in terms of support, you absolutely can't afford complete vendor lock-in.

Especially when the vendor holds all the cards (e.g. Android).

(Why this doesn't pervade into desktop operating systems, virtualisation platforms etc. in some companies is the real question that keeps me awake at night).