How do people have so many papers in just one year?
A few remarks:

Those who write 20 papers a year typically write with co-authors. At this level of productivity, the authors often take over a part of the research and writing for the paper. More senior authors often do not conduct the experiments, but rather have a share in designing the overall idea developed in the paper and a part of the writing process. If you are really on your own and cannot join a team (e.g., take over a role in some other project work in your research group), then you can't expect this level of productivity from yourself. And ultimately, it's also not needed.
If your advisors dismiss your ideas as too simple, there are four possibilities:


the idea is actually quite simple, and others have tried it out and published about it
the idea is so simple that it's non-publishable
the idea is simple, it has been tried out, and the results were bad
your advisors are wrong

I've seen cases where the second case held, but later another researcher tried it as well and successfully wrote a paper about it.

About paper rejections at conferences: In my experience, the quality of reviews varies greatly between the conferences and the reviewers. Sometimes, reviewers take a lot of time to write precise reviews for why exactly they recommend a paper for rejection. These are very helpful reviews and should be taken very seriously. But then there are others in which for clear rejections the reviewer does not take the time to write up why exactly the paper should be rejected. Quite often, when the answer should be &quot;the paper is overall not written well enough&quot; or &quot;I don't believe that the result is significant&quot;, the reviewer singles out something that is easily defensible objectively, such as a missing comparison against something else, missing references, or the like, and  bases their rejecting recommendation on that. The problem with these reviews is that while the reviewer is normally correct (but sometimes isn't), it doesn't help the author to address the core problem. Normally, your advisor should spot such reviews and tell you what to focus on, but if you get little help, that's not good.
If you see other published papers that are similar to your own ones, read them and then think about why they were accepted. What is the core contribution that the paper made? What is so cool about it that the paper was accepted, despite its inefficiencies? It may simply be that for the papers you are submitting, no program committee member has a good reason for why they should make a case for you that exactly your paper should be accepted. And then it is rejected even though it's not actually bad on any scale.
Solicit feedback from peers (fellow PhD students) if your advisors won't give it. Don't ask peers about feedback on grammar, spelling, and the like on your papers. Ask if they would be writing an &quot;accept&quot; review for the paper or not. Any why they would do that. Get feedback on the paper story and if the argument and structure is convincing.