Why professors spend most of their time to write proposals than doing research? I have been working in academic career in several years. I have noticed that almost all professors spend most of their time to write proposals?

And to my surprise, they are not working on research papers. This is reflected by the fact that the number of technical papers with first author (single author) as a senior professor is decreased quite dramatically.

My question is, therefore, is there any hidden (but valid) reason for this?
I'm a bit puzzled by the word "hidden," since I don't think the answer is at all hidden -- in fact, I often discuss this with my students. Why spend lots of time writing proposals?

(1) It's expensive to run a lab! At least in the U.S., a graduate student typically costs ~$50k/year, which includes tuition and overhead costs that the student doesn't see. Postdocs are ~$100k/year. Equipment and supplies are not cheap. The sum of all this is a lot of money.

(2) For a variety of reasons, it's hard to get money. The success rates for proposals at NIH or NSF are around 10-20%, meaning that most proposals don't get funded. Certainly more than 20% of proposals are very good -- at least based on my experience on grant review panels. Writing a proposal that's at least very good takes a lot of work. Writing N such proposals so that enough get funded to keep one's lab afloat takes an even larger amount of work.