Apple propaganda notwithstanding, the reason tower PCs are big isn't because they're outdated. The reason tower PCs are so bulky is because they're designed to be user serviceable. The case has lots of open space so your big, meaty hands can easily access all of the components, and everything is secured with friction-fit tabs and standard machine screws to minimise the need for specialised tools. A properly laid out tower PC is fully serviceable with a single Phillips-head screwdriver and no greater manual skill than your average Lego playset – heck, for some of the more modern case layouts you don't even need the screwdriver, unless you're performing major surgery like a full motherboard replacement.
Like, think about who benefits from convincing you that a fully modular computing device that can be serviced and repaired with your bare hands and minimal technical skill is unfashionable.
Back in the day, I used to build my own PCs. I'd run down to Fry's Electronics, pick out a case, a motherboard, a CPU chip, memory, and whatever other components I needed. I'd drag it all home and assemble it myself. I'd load the O/S, power it up, and viola!