Want a legit answer? They lack power isolation. I had a device surge/short while attached to a USB hub (it was a branded one), and everything else attached died: Keyboard, Mouse, motherboard, and a phone.
This was back pre-USB 3, and the power limit of USB has only increased since then (USB 3, USB-C, etc). This means the wires get hotter, the hub gets hotter, and any short/surge will have an even more dramatic effect (e.g. fire?).
Honestly USB hubs should be up to the same spec as a surge-protector/multi-tap, but they aren't, even though they carry enough electricity to kill someone.
Nothing is wrong with hubs, but now there is yet one more thing you have to have available (at least until they become more prolific in external devices)