Because it's a tool for informing doctors, not a medical treatment. We don't have a clinical trial before PubMed rolls out a new search form either.
BTW I've worked with regulated software and it's not pretty. Just to update the firmware you have to fill out a ton of paperwork (because somebody died once after a firmware update and the firmware update was found to be the root cause). Regulation and review doesn't really make things tremendously safer, it just provides an audit chain so you can point at something when it does go pear-shape
Laws are changing to include "software medical devices" in health regulation. The FDA recently recognized this as a gap.
It's important understand that FDA approval to market is always done in terms of indications for use. You don't approve a device full stop, you approve it (and evaluate it) in terms of the indications. Only in this context can you determine if you have a 510k path (i.e. short cut) or need a PMA (i.e. clinical trial). It is also important to realize that different panels in the FDA (i.e. the type of device you are making) have different evaluation criteria.
So "Watson" is unlikely to ever be cleared per se (it is a tech platform) but you are likely to see individual Watson enabled systems file 510ks for particular indications. If I recall correctly this has already happened.
While I get the impression FDA and industry prefer 510ks as much as possible, personally I don't see how this is close enough to anything out there right now for that to pass the smell test.
My guess would be that some `Watson` equivalent stuff will be 510k'd in as part of an MRI, CT or DBT imaging system or a CAD tool working on these systems. I think it'd take a while though, I don't think the regulatory science is there yet for actually showing value.
There have been approved CAD systems separate from imaging systems since the 90s.
"Value" is a nebulous term. FDA looks specifically for "safety & efficacy", and the criterion for each are rather dependent on the intended use.