I guess it makes sense to standardize to a few sizes but why are SSDs made in a 2.5" form factor? Why not make them much smaller - essentially just large enough for a SATA connector and a power connector? This would make it easier to make more compact laptops and to make very tiny HTPCs in a mini-ITX form factor. I know that there are some custom form factors and there is a Mini PCI-Express version but why not have the most commonly sold versions be a smaller size?
The reason is to make them compatible with existing systems. SSD on board on some latops are very small (looney/twoney) size. Some onboard SSD don't have a typical SATA connector. They can be connected via ribbon cable or using PCIe ports on the motherboard.
mSATA are what you guys are thinking of - they essentially made a new SATA connector to fit these tiny SSDs
(some are PCIe direct connect, but rarer)
As for the OP, it's simply for standards.
Many laptops are slide-in vs top-mount when putting in a drive
If you make it just the length needed, and width of a SATA-interface, good luck getting that one in and out
(for a desktop, no problem provided you go to the first standard screw hole spot, so you can actually fit i in a standard case properly)