The transition engines had the drain plug undrilled and unthreaded, but later models the drain plug boss was no longer in the engine pan.
Remember that engine moulds cost hundreds of thousands of dollars to make so no one is going to scrap them till they are worn out .
And with exotic metal moulds as used in modern casting machines that can be a very very long time ,
Add to that the equipment used to make the moulds is also very expensive so when saving every cent counts no modifications get made till it is absolutely necessary which would usually be when a new model replaces the old one .
As for saving a few cents, yes that is exactly why it is done because YOU the mower buyers have an unrealistic idea about prices so mower makers are forced to cut costs where ever possible.
If Joe Public bought on quality then 1/2 of the people would be using Honda's and Honda would still be making ride on mowers and Honda would still be the dominant top end mower engine in ride ons.
But the public has been brain washed that they can have it all for nothing so buy cheap.
And as dad used to say
"he who buys cheap pays thrice "
I am currently doing tyres on a Greenfields mower that is 42 years old for a couple who are in their 80's .
They bought it when they bought their 2 acre property 42 years ago and it will probably run another 42 years .
It was 2 times the price of all of the imported US mowers when they bought it but has worked out to be 1/4 of the price of buying one of the heavily advertised US imports none of which would have lasted this long.