It's going to be precisely the reason. If you have a dedicated wire, fibre or copper then the entire available bandwidth is available per connection (one caveat with copper is crosstalk but it is minimal and can be mitigated). With fibre the available bandwidth per strand is huge.

It's so fast that even where there's contention, it is rarely a problem that everyone sharing a part of the connection is downloading or uploading at once. So pretty much most of the times you test, you get the full speed.

With mobile data, the entire cell is sharing a small amount (in comparison) of spectrum. Unlike a wire, the entire spectrum cannot be used by a single tower, a pretty small number of channels are carved out for them. Also because the signals are travelling through the air, there is more of a problem of signal loss and interference to contend with, so the channels very rarely reach the maximum possible speed (forward error correction and reducing bits per symbol to reach a suitable signal to noise ratio both will reduce speed for example.

For upload (which isn't usually much of an issue) there's another problem of guard time between timeslots. When downloading, the cell transmitter transmits the whole time and shared the channel between all users (another thing that can slow things down) so there's no problem of needing a guard time. But when it's separate transmitters (phones) sending there's going to be a guard time between different handsets timeslot and the more active transmit stations there are (phones) the more these guard times add up to wasted bandwidth. Luckily most people are downloading far more than uploading, so it's less of an issue.

I think for these reasons caps are used to limit people from ALWAYS consuming data on the cell/mobile networks and instead using wifi wherever they can in order to keep it fast for those that do/need to.