The customer base is very large companies who find having someone to sue reassuring and are likely to be institutionally incapable of running their own fork of java. Seems fine to me.
Well there is also the fact that the openjdk codebase is huge and so even though it is quite well written and documented fixing a bug in the jdk (and especially a bug in hotspot) isn't easy. Although tbh i have no idea how the quality of support is with Oracle.