I can answer this: because people do not want to pay for a developer, they want to pay for a designer. (It is exceedingly rare to find both in a single person.)
The designer can, within the structure and strictures of Wordpress, build a site with all the whizzy things the client wants, and they can charge $X for it. A developer can do all of that as well, but they will probably need a designer as well (see above parenthetical), and it will cost $X * 2.
The counter to that is, "well, just build a good templating system and tweak that for other clients, and you're good." Which is true, and that's why there are lone PHP developers out there who make a decent living with a stable of clients.
One of the worst aspects of Wordpress is dealing with the infuriating and opaque reasons why a particular site is slow. A relatively simple site can be maddeningly slow for any number of reasons, and finding them is like searching for a contact lens in a full bathtub.
If performance is a real business concern (and when is it not?), hire developers, not designers.
Hire developers, not WordPress developers. Because the latter will push WP onto a project where clearly the downsides, like performance, outweigh the benefits. But the latter may pick WP when it is a good fit.
I was with you until the end there. Tools for performance monitoring exist. Query Monitor is a great starting place for a free plugin. Lighthouse will tell you if you have a million blocking requests. New Relic is offered by any web host worth a damn and WP plays very nicely with it.
Fair enough, but you just listed three different tools, and you may have to go through all of them to find what's going on, and come out the other end just as lost.
I mean, I get it. I've written some stuff that screws the pooch performance-wise, and the solution isn't immediately clear. But with WP, and with enough installed plug-ins, the reason for the slowness may simply be "in order to work with WP, there is a lot of back-and-forth with the DB amongst all these plug-ins," and there ultimately isn't anything you can do about it.
WP is amazing that it does so much and works as well as it does, but let's not pretend it's not a bit of a faff sometimes.
The designer can, within the structure and strictures of Wordpress, build a site with all the whizzy things the client wants, and they can charge $X for it. A developer can do all of that as well, but they will probably need a designer as well (see above parenthetical), and it will cost $X * 2.
The counter to that is, "well, just build a good templating system and tweak that for other clients, and you're good." Which is true, and that's why there are lone PHP developers out there who make a decent living with a stable of clients.
One of the worst aspects of Wordpress is dealing with the infuriating and opaque reasons why a particular site is slow. A relatively simple site can be maddeningly slow for any number of reasons, and finding them is like searching for a contact lens in a full bathtub.