I'm going to assume that this just because Steve hadn't had the time to include missile launchers as STO weapons and/or there being something in the code for missile launchers that makes things lot more complicated - like magazines for example.
But yeah, ideally we would have both missile silos as an installation and STO units that wield missile launchers and supply vehicles that act as missile magazines.
STOs have integrated BFC limiting them to the range and purpose expected.
Assuming you are to introduce missiles to STOs how would you handle that? This will require an extra slot to add an existing MFC or a brand new one integrated.
Furthermore what about missile size? Existing or integrated.
Ultimately ordnance, you will require another slot to choose which missile is being used.
All the above will require a brand new mechanic that cannot be handled by STOs in their current form and, but this is my opinion, the problem is already being addressed by Stations. I also think maintenance is a fair price to pay for something that would be otherwise too powerful free of charge.
You can potentially tell that this is the way (Tha Mandalorian TM) it has been currently thought by looking at the
precursors which use missile stations as AMM ASM planetary defense platforms.Obviously, everything can be changed/included don't get me wrong.