Connect my 2017 MacBook Pro via ethernet cable to Apple Airport Extreme (tower)

You have turned Off the WiFi on the MacBook Pro, correct? If not that would be the first thing that you would want to do, since the connection will very likely remain over WiFi, even if an Ethernet cable is connected.......because the MacBook Pro prioritizes WiFi over Ethernet.....unless.....you have manually changed the Connection Order preferences on the Mac.

If I did that, I would have to access my Airport Utility elsewhere.

No, you wouldn't. You can access AirPort Utility using the Ethernet connection, as I am doing right now. This will happen automatically if you turn off the WiFi on the Mac. Close AirPort Utility first, then turn off the WiFi. Then open AirPort Utility.

And just curiously, why can't there be both Wi-Fi and ethernet available on the same computer?

Both WiFi and Ethernet might be connected at the same time, but only one of them will be the active connection at any given time. As I stated already, you can change the priority of the connection type to tell your Mac whether to connect to WiFi when both connections are in place, or whether to connect using Ethernet when both WiFi and Ethernet are in place.

In the example below, you can see that my Mac prioritizes Ethernet over WiFi if both connection types are in place. I'll bet yours does not unless you have manually changed the Service Order on your Mac.

Older Macs prioritized Ethernet over WiFi. Newer Macs......those that do not have an Ethernet port.......prioritize WiFi over Ethernet.

Bottom line......the only way that you can know for sure that Ethernet is the active connection is turn off the WiFi on the Mac......because the Service Order option has been known to have some bugs in the past. Not sure about now though.

Once again, you can have two connected at once, but only one of them will be active.

If you are referring to choosing between WiFi and Ethernet in AirPort Utility, that tells AirPort Utility how to connect.

BUT.....your Mac will connect to the Time Capsule using whatever connection is at the top of the Connection Order list.

Check the Service Order List to find out how your Mac is transferring data from the Time Capsule to the Mac and vice versa. This is nothing to do with AirPort Utility.

If you find that WIFI is at the top of the Service Order list. (It likely is) then your data between the Time Capsule and Mac is being done over WiFi, not Ethernet.....even though the Ethernet cable is connected.

Post a screenshot of your Service Order list if you are still confused and we'll tell you how the Mac is connecting to the Time Capsule at the moment.

Does it look like this?

Or does it look like this?

Big difference

And if I want Wi-Fi back, I just turn it on. (I've redacted the numbers)

And, if your network preferences look like they do above, the connection is still over USB/Ethernet, not over WiFi. Even though you have turned WiFi back "On".

If you want WIFI to be the active connection, it must appear at the top of the list, not second in line.

Life would be probably easier for you if you adjust your connection priorities using the Set Service Order option. Using that option, you can have both WiFi and Ethernet connected, but the active priority will be Ethernet.....or.....you can have both WiFi and Ethernet connected, and the active priority will be WiFi.

It's your call.