It's cyclical. There have been segments of golf history where the average length of a professional golfer played the average length of championship courses perfectly, be it the gutta percha era, the Dot era, the smaller ball era, or the balata era. One can also look at the persimmon era. Then there were always freaky players like Nicklaus & Tiger who come along & reset the game. But it's gone too far now in favor of average length of player v. average length of championship golf courses. It's simply becoming untenable.
The courses the USGA uses for example for the US Open rota are typically of an older era which can only be lengthened to a finite amount, if at all. The two newer courses in the recent past have made them go gun shy due to the debacle at Chambers Bay & the decidedly un-US Open at Erin Hills. Look at the upcoming sites, it's obvious that a slowed down ball was in the works (
https://golfweek.usatoday.com/lists/u-s-open-future-sites-through-2051/). It typically takes years or decades to become a course to be a major location for many reasons. Kohler has performed the best of these newer courses, but it was also designed at the nascent period of hotter balls & bigger drivers & Dye understood he needed to lengthen it out to 7800ish yards to cope.
e - to widen the scope, consider the Florida swing & how old the golf courses are on which the tourneys are played plus current length:
Honda - PGA National; built in 1981; 42 years old; 7048 yards
Bay Hill - Bay Hill; built in 1961; 62 years old; 7300 yards
Players - TPC Sawgrass; built in 1980; 43 years old; 7245 yards
Valspar - Innisbrook 'Copperhead;' built in 1974; 49 years old; 7340 yards
Average age & length of the Florida swing courses is 49 years old & 7233 yards. These courses can only be lengthened a certain amount & I would guess most of them are tapped out, especially Honda & Bay Hill (to a lesser extent, Copperhead) as they are routed inside gated communities often with houses on both sides of the holes. There's no plans for any of these four tournaments to move from these courses for the foreseeable future due to infrastructure, logistics, & tradition. But they are basically too short these days. They would need to be each three hundred to four hundred yards longer to be in sync with the current length of the pros.
These courses are on the cusp of being virtually obsolete, veritable pitch & putt courses for much of the field. The only way to legitimately make the courses more fearsome is to reduce the yardage off of the tee. The choice was to tone down the golf ball & leave the clubs alone in the meantime.
Tl/Dr: the ball had to be slowed down to allow for the courses to be played with a semblance of what the architect designed for the course.