Well it goes like this.
Think about how much those engines costed back then and what you earned back then so you can convert the cost of the engine into days of work.
Do the same for a modern engine and now you will suddenly see that the modern engine is around 1/2 to 1/3 of the REAL COST of the engine not the dollar number.
So those older engines were a lot more expensive and there was a lot more profit in making them so they could be made substantially better.
Next we have computers now days and a good operator can model changes in tiny incriments till the part "fails" on the desk simulation so you now get an engine with the thinnest lightest parts that will last the designed service life when looked after properly.
Back then the physical parts had to be made,assebled into an engine then the engines run to destruction, pulled apart, analysed, improved , modified whatever then the new part assembled into another engine and the whole circus starts over.
This costs a small fortune to do.
Thus it was cheaper to over engineer the engines.
This applies to all "Consumer Durables" from car to tooth brushes.
Nothing last as long as it used to unless you buy the absolute top shelf item.
However most of us no longer have the skills to evaluate what is a top shelf item and our decisions get peverted by things like advertising.
AS I have said before, back in 1964 my father paid more ( in hours of labour ) for his base model push mower than most of the big box ride ons cost today.
His old push mower is still running perfectly and the only major part I have replaced is the piston rings once.
My sister still uses it to mow the common grounds around the block of units she lives in.
Even if you knew your push mower will last you 50 years would you pay $ 2500 for one ?
The answer is no you will pay $250 for one that might last 5 years then go shopping for another one when that one starts to play up.
I have a theory that one of the reasons we are so time poor now days is that we will no longer pay for top shelf good so we need to continually replace them and in the case of a mower you can spend more time looking and deciding what to buy than you will using it for the next year if not longer.
Think about it.
The average mowing time down here on a 1/2 to 1/4 acre residential block is 30 hours a year