[taejo]

You may be speaking across purposes, just because "thesis proposal" means different things in different places. In some institutions, it's a proposal for a whole thesis project, from experiment design through conducting experiments to drawing conclusions and writing up. In other institutions/countries, it's done as the project is drawing to a close, with experiments completed and data gathered: the candidate then proposes the thesis that this data supports, which they will write up and defend. The latter is more of a formality, because it happens at a different step in the process. The point where innovation and ambition incur risk has mostly passed by then.

[analog31]

In my case, the thesis proposal was at the beginning of an open ended and fairly long term project. You were supposed to conduct a literature review, show that the project had a good chance of working, and that you had a good enough grasp of the subject matter to handle the work. It was also a point where you could opt to finish with a Master's instead, having completed your coursework.

There was high attrition in between this step and getting the PhD.

The other end, where the student is basically done except for the writing, is less risky. One reason is that professors are strongly discouraged from letting a student fail a thesis. When I was ready to defend my thesis, I had to get permission from my committee to do so, and their signature indicated that they considered my work to be defensible. So the defense was in fact a formality. Failures at that point, that I know about, had to do with students getting caught fudging their work.