Maxperson
Morkus from Orkus
Do we know this for a fact? Living tutors didn't seem to stop the Jedi Order from becoming sclerotic and producing the weapon that would ultimately bring them down. A living tutor didn't prevent Ben Solo from being corrupted by a Dark Force outsider. Maybe it's time for a completely different approach.
Look at what Yoda did in training Luke. Nothing that we saw him do needed him to be there in person. He could have guided Luke, demonstrated forms, etc. as a force ghost. He did sit on Luke's back as he ran, but he could just as easily told Luke to tie a log onto his shoulders and proceeded through that portion as a force ghost as well.