There are 3.5" SSDs available, but they are not common:
The 2.5" form factor presents a lot of benefits when compared to 3.5" ones: they are smaller, lighter and cheaper (require less materials to manufacture such as aluminum and plastic). Even though, 2.5" generally offers the same features such as speed and data capacity like the 3.5" ones.
Also, thinking of HDDs (this does not apply to SSDs), it also has increased energy efficiency (less power consumption):
So regarding costs and benefits, there are no reasons to keep SSDs (even HDDs) in a default 3.5" form factor, but if you really want to install a 2.5" SSD or HDD into your desktop, you could just use an 3.5" adapter such as this: