Even that is problematic for them to do
It was brought up how bad the licensing chosen when transitioned from Oricle to Apache was announced, it was set up in a way they can not easily include LibreOffice's improvements but LibreOffice can include most anything from OpenOffice other than things like trademarks of OO. Granted one of the reasons for the split was there was a ton of third party improvements that had not been merged into the core code base and they did a lot of clean up too, so it was likely going to be a long time before OpenOffice would be in a position to be doing work that was not already done on LibreOffice even if it had gone well and attracted a much larger set of devs.