Most drive belts i've seen don't have the retainers. Just deck belts.
Your mower is a Cub right?
What Brand of belt did you use? If it was a Stens, they're junk.
You need to get out more often and look under a few more mowers.
EVERY BELT DRIVE SYSTEM THAT CLUTCHES NEEDS A BELT RETAINING SYSTEM OR THE DRIVE BELT WILL RUN OFF THE PULLEYS WHEN IT GOES SLACK TO DISENGAGE THE DRIVE
That should be blindingly obvious to any one with basic understandings of belt drives.
On the Cub 1042, most of them are intergrated into the design of the pressings.
Note part # 783-06260-0637 it has 3 sides turned up and they are the belt retainers.
Note part # 783-06027B it also has a curved edge turned up near the spring anchor which is the belt retainer for one of the V idlers part # 756-04325
The second V idler, part # 756-04325 is retained by the side of the chassis.
Flat idler pulley , part # 756-04224 has the belt running around it also retained by the stepped sides of the "Bracket, Idler, Center Double " part # # 783-06027B.
So some clever engineering has done away with the single item loose belt retainers found everywhere an products from the AYP factory but all drive belts on tractor style mowers are retained.
As for your blanket statement about Stens belts , about as crediable as your statement about no retainers.
Been fitting their OEM ( 265 series ) belts for 4 years without a single problem that was not my fault .
Most of these are made by the same factories as Cub belts, on the same machines with the same cord placements using the same compounds as the belts Cub fit .
OTOH if you fit the std cross section 328-248-258 series belts because they are 1/2 the price then they may have given you grief as Cub use non standard belts.
cpurvis was 100% correct, if a belt shreds on start up it has been fitted wrong or there is broken &/or jambed pulley, or a bolt protruding and rubbing on the belt or something similar.
Usually when a belt shreds upon sart up it is routed wrong ( we have all done it ) and is running on something that damages it.
FWIW I specialze in 2000 series Cubs and fit Stens belts to all of them which work quite fine.