Because you'll miss your target? Allied bombers used to go out in 1000 aircraft formations because that's what they needed to do in order to hit their targets. A couple of dozen aircraft aren't sufficient to either terrorize enemy civilians into submission (something that 1000-aircraft* Allied firebombings were never able to do, anyway) or hit enemy military targets. If the Russians are using dumb bombs, they are delivering more airstrikes than Assad could on his own, but they are not giving him a capability he doesn't already have.
* Each B-17 might carry 8 500lb bombs, meaning that a 1000-bomber raid would result in 8,000 bombs being dropped in a matter of an hour or so, vs the dozens that can be dropped from the few Russian aircraft deployed.
Modern avionics have improved targeting for dumb bombs far beyond what B-17s and other bombers with the Norden bombsite could do in WWII.
The Israelis took out the Osirak nuclear reactor in 1981 using eight very early model (A model Block 10s, i think) F-16s with dumb bombs.
The Russian dumb bomb targeting is going to be at least as advanced as that.
The current accuracy of gravity bombs the Sovs are using far exceeds the capability of WWII systems, there is no comparison.
But less than 10% of those 1000 aircraft x 8 WWII bombs per aircraft (on a big raid, maybe every third day at most) with 10 crew per bomber + 5-10 ground crew ... But less 10% fell within 500 feet of the target.
By the end of Reagan's buildup of conventional forces/start of Gulf War I, we'd be able to carry more weight from an A-10 or A-6 or A-1, fly three or four missions a day, keep the aircraft over target for hours between re-arming trips with air-to-air refueling, and use only 1 bomb per target with a 90% HIT rate on targets as small as a single tank. And an A-1 or A-10 only needs one pilot.