Both their functionings are complex, so people can get impressed. But for both of them, all the complexity is inside the device and there isn’t much to put together; and the way they hook up to your house is really simple.
Why YSK: so you don’t live too long with a broken toilet or garbage disposal, thinking it will be too hard to replace. Those two are some of the simplest things to DIY.
In fairness, the hard part of replacing a toilet isn’t the replacing of it, it’s moving it about, and having to deal with the mess.
A plumber is usually better equipped for that, and would know what to expect, whereas someone who is less experienced might end up destroying the toilet and/or splashing water everywhere (if they have a siphonic toilet), or breaking something else.
A tap is probably easier to replace, by comparison.