Well what do you want ?
When you engage the blades you put a load on the engine so the speed changes.
If it goes so slow that the deck does not cut properly then you need to have the governor adjusted.
You never mentioned that the cut was bad so it is assumed the cut is fine.
As for the belts & pulleys
The blade activation and deactivation requires the belt to be tight to drive the blades and loose to allow the blades to stop spinning.
If the bet was running on the very outside edge of the pulley when the belt goes slack it would jump off the pulleys, so the pulleys are deeper than the pulley for an electric PTO.
Every yea the actual price of mowers goes down because the buying public are greedy, selfish & cheap
THus whoever makes the prettiest mower with the biggest engine for the lowest price sells a lot more mowers.
Then you add the retail control where large retail chains dictate how much the mower company can charge them for a mower.
A section belts are 2/3 the price of B section belts so by putting an A section belt on the mower MTD saves $ 3 on the belt and $ 5 on lighter pulleys.
So your answer about WHY is CHEAPNESS pure & simple.
Purchasers would rarely ask about the deck belt and if the light belts break more often then MTD will make more money selling more belts.
It is all about money & peoples refusal to spend it,