>It takes quite a lot of time to prepare talk and it is not like they would pay all that impressively.
Most tech events don't in general pay for speakers or any travel. (Keynotes, scholarships, etc. are among the sometimes exceptions.)
I rarely literally re-use a presentation slide for slide but many presentations I give are based on an earlier presentation I gave. In most cases, a different conference is going to be a mostly different audience. In general, it doesn't make a lot of sense to just dump a presentation because I gave it once to a couple hundred peop,e.
So I think that repeated talks and intro talks are to be expected. Through I did not mind them, I used them to get idea about new thing X while I have time set aside for learning.