For different reasons.

Vitrectomy has serious risks in very low percentages, but it has risks. On the other side, until very recently, floaters are teach in medicine schools as something benign that once you habituate to them, they don't interfere in your quality of life. Of course we sadly know in many cases this is not true, but is what they have learned as the truth, so they don't see the reason why to expose the patient to any risk if he just have to wait to get habituated to floaters and have his problem solved.

Another reason is many doctors are not updated about the complication rate of the modern vitrectomy techniques and they still remember the surgery they studied and the risks at the time (this applies specially for the ones that aren't not vitreoretinal surgeons so they don't operate). Obviously a year 70's or 80's vitrectomy was that risky that floaters would be the least of your concerns, now this changed and in fact updated vitreoretinal surgeons point in this direction, but that's just the vanguard, years would be needed to change the mainstream's opinion.

Another possible reason is that they think they help you to get habituated to floaters if they deny you have other possibilities. In fact this trends to create anxiety and depression, because it's easier to cope with something you know you can get rid of if you can't make it anymore, but for some reason they see it that way.

Then there are also MDs (in all specialities) that think you should avoid surgery at all costs. There are also Doctors that see really bad eye problems every day so they think in comparision floaters are nothing... they are humans, every one with their own opinion....

That's why I think the relevant information source are the research papers on the matter and not one individual's opinion.