Why not? You bought a cable for every monitor you'd ever connect to. The hdmi cable your monitor came with definitely didn't plug into your MBP from 2015.
While true it's beside the point that was made earlier on. The Parent complains about "dongles", but really it's not much different than having to plug a portable computer into all of the accessories that are there already, there's just something extra tacked on. It's not like other vendors and set ups are immune to this; my previous work place had a Dell contract and their desktops came with some stupid video card where there was only one out-put and the only way to get dual monitors was a Y splitter from Dell.
In custom setups, if your card isn't Dual [connection], you use an adapter for DVI or HDMI, and so on.
Apple does have a lot to answer for, and their idea of "dongle for everything" does get tiresome and Apple should be above that. But it's not like it's unique to Apple or even new in the computing world. This has been standard for a long time.
I used to work at a place with a Dell contract, and I had a laptop ...with a docking station.
I always pooh-poohed proprietary docking stations before I had one -- all the cables went to it, dock the computer, done. Best of all, it wasn't having plug something in where you need to set the computer down, then navigate something with one hand -- the dock felt nice and intuitive to set the laptop down upon with two hands, as if it was meant to be part of setting your computer down. Even worked flawlessly with my Linux install.
Sadly the E-series dock was not as intuitive -- I found it easier to manually plug in my headphones as the dock was a different sound device.
It's funny because there isn't a single non-standard interface on the new MBPs and the old ones had magsafe (apple only) and thunderbolt (might as well be apple only)