It is hard to design USB well, particularly with respect to power: a *huge* thing is making sure it's safe- Lithium batteries are dangerous when charged wrong, and if there is a fire, the lawyers will be after everyone they can possibly name in the suit. Remember that the lithium battery is very energy-dense- a lot of energy in a small space means the potential for a lot of heat in a small space.
All computers have some method of limiting the current out of their USB ports- if they don't, they can't get a USB Logo. During enumeration, a device requests more current, and the computer keeps track of the current available. If the current isn't available, enumeration fails. If a device draws too much current, the computer can crash, as it will drag the computer's 5V rail down. Most computers have current limiting in the form of a NTC resistor that will limit current but only after it heats up, so there is a delay, so short term overcurrents that aren't long enough to heat up the NTC resistor are dangerous. USB relies on the devices following the spec. If you violate the spec, you fail to get USB logo- and many of the big OEMs require logoed devices.
There are many USB hubs that can natively support more than 4 ports: Microchip's USB2517 is one (of many) I'm familiar with.
The 100W devices are coming as part of the USB C Connector, but with all that additional power, you better believe that the computer manufacturers are going to be careful as there is a much bigger chance of fire. To even get 100W, you have to have an active cable that identifies itself to the system as one that can handle the increased power. And Apple is very involved in USB type C development.