Like many have said, it is not a matter of one or the other. I’m building a large layout and it needs lots of rolling stock
For me, I like to build models, so I often chose kits if they suit my needs,
But I have lots of RTR stuff as well - EXCEPT, that virtually none of them make it from the box directly to the layout - most get my “special” trucks, all get REAL Kadee couplers, and most get some VERY LIGHT waethering.
Also, while I like detail and reasonable accuracy, I gave up the rivet counting thing decades ago. A nicely weathered Athearn Blue box model is just as welcome on my layout as the latest B&O caboose from Spring Mills.
AND, because I am one of those “freelance” modelers with my own roadname, I always have an eye open for undecorated models that my home road roster requires.
I build craftsman kits from wood/metal/resin, I build plastic kits of every detail/skill level, and I buy (and modify) RTR of every price/detail range - because my primary goal is to build a model railroad witth a very specific theme.
AND, I have a fairly large roster of older models, Varney metal kits, Athearn metal kits, Silver Streak wood kits - all still running and looking just fine among the newer models.
As for why which shop has what, or why manufacturers are doing what they do, I’m bored with those discussions - I will only say this, I’m glad I have most of the equipment I need for the theme of my layout.
Sheldon