Yes, althought in the case of wordpress, it's also because it's really badly architectured: no internal cache, so many requests for just one page, comment are loaded even when you don't read them, etc.
The first time I peeked at wp code I was flabbergasted at how cavalier they were with piles of inefficient queries.
At the time I was tasked with evaluating it as a "cheaper" replacement for an expensive, managed CMS. My conclusion was it would end up being more expensive to accommodate all of this inefficiency with caching galore not to mention support and dealing with a loss of dynamic web sites.
I looked again last year, assuming it had greatly improved, had replaced all that with GRPC or graphql and nope, still horrendously designed.
I wonder if it's intentional or just a case of support for backwards compatibly.
Optimising WordPress is a known and solved problem. I don't mean to undermine your comment but it is trivial for a web infrastructure ops guy to deliver on. There is a lot of fud out there on the topic so it's easy not to see the diamonds in the rough.
That's not the point. The point is it's not built it, because the architecture decisions were unsound.
Now, don't get me wrong, I use and like Wordpress. There is not more versatile yet approachable blog/CMS/whatever out there, the ecosystem is huge and it's my go to tool when I want a quick and dirty solution.
But it doesn't change the fact it has been badly designed from the start.
Popularity in software doesn't reward good design. It rewards solving problems.
It's not optized at all.