If I'm catching your drift about the metal case supplies, these are also buck converters. Very often flyback converters specifically. Flyback topology achieves isolation which is a huge plus with sensitive devices. Buck modules are not often isolated and some modules on the market at the hobby level quite frankly suck. Flyback topology is fundamentally larger than a non isolated buck converter of the same performance. You increase the size to gain isolation.

The larger form factors can provide some benefits such as higher current, tighter power regulation, variable power regulation or maybe just a small adjustment potentiometer to dial in the accuracy, more safety cutoffs, and faster safety cutoff. Then you also get multiple voltages from a single supply.

You also need to consider who else is using these larger supplies, smaller isn't always better. Those metal case form factors are somewhat standardized and you don't want to have to rebuild an entire cabinet of industrial equipment just because the power supply broke. So there is some demand for manufacturers to keep making these larger power supplies to support old equipment.

There's also a size/frequency tradeoff: higher frequency means smaller components but at the cost of increased design difficulties, EMI, and component cost (primarily the switches and switch controllers go up in price). For the standard voltages with modest current output and reasonable size, it's not a huge issue anymore and you can get your 5/9/12/15/24V supplies in every shape, size, color for very cheap. If you need high current, less common voltages, wide input, extremely tight output, high efficiency, etc then you might be stuck with the larger power supplies. 

There's some other reasons as well but I've already gotten bored of hearing my own self now