Why is it so? Why are they content with poor fingerings which result in poor articulation (legato or articulate legato)?
Regardless of what they say ("I don't have the time", "I will manage to play correctly without writing in the fingerings"), there are only two real reasons:
1. They don't care enough about the result to practice correctly (their problem).
2. They don't trust the person who advises them (your problem).
The only way to help such people change their minds about the fingerings is to earn their trust first.
And sometimes it's OK to simply say: "It's not for you."