(Broad brush applied by Captain Obvious)
The problem is the "cleaning up" of the stuff that "gozinta" the newer engines and the elimination of things like lead, yellow metals, oil based sealing components/rubber and such things.
The "newer" stuff, often leaches out or attacks those materials present in the older engines, and/or those additives and chemical compositions that worked with prolonging the life of the older engines and causing problems.
It is not often as critical on a show car, trailered or only driving a few hundred miles a year as the fluids are likely kept fresh and changed. But hard driven or "daily drivers" (like mine), have the needs of many of the older make up of fluids etc.
Copper, bronze brass (yellow metals) in radiator, synchros other gearbox components, and bushings coming in contact with some of the high detergent lubricants and synthetics* that also have ZDP and other chemicals absent, can leach out those metals and some of those metals by virtue of their alloyed makeup also provided sacrificial wear/lubrication and therefore protected the harder metals, and the "new juices" often negate or compromise that function.
Others attack lead in solder, and or the liberation of metal components and the suspension in the fluids can result in galvanic corrosion or dissimilar metal corrosion. Most of these things are very slow acting, but even though slow, the effects are permanent and cumulative, so often go unnoticed until something fails.
Most of manufacturers (unless focused on classic cars) have no concern for any of this as it applies to older cars.
And they make changes that make their product profitable, desirable and affordable for their markets.
* synthetics, btw are mostly the same beginning as "natural/mineral" oils, but they are specifically formulated hence the word "synthetic", but the term is misleading to some that the "lubricant" is not truly "oil".
When it cam about, it was at a time that lubricants were pretty much base refined and nothing more. The a few additives became normal, and then a giant leap with "highly formulated" oils, and the distinction and some of the marketing hype coined the term "synthetic", as a "superior" (and misleading, man made = better), but it is the term that is (mis-)used.
Most know of the issue with eliminating lead in fuel, it was a "big deal" (at the time) back in the late 70s and 80s, and many feared their valves were going to fall out of their tailpipes after one tank of gas. Some cars did suffer valve seat damage, some were high nickel or otherwise harder material and some engines already had hearned seat inserts.
Valve rotators helped, and the fact the leaded fuel continued with at least a gram of tet, helped prolong the needed protection of lead.
There are also benefits to the new formulations, so it's a trade off, but if folks do a little homework, you can stack things in your favor by selecting the best things to put in, and the worst things to leave out.
And realize that there are cars out there a hundred plus years, still running, and many cars that are 50 years old as daily drivers. It just takes more effort and knowledge than when they were new and almost anything was good for them.
