Because that is not relevant to a player’s value. Since Mays and all his teammates and opponents travelled the same way, that effect is the same for Mays and all his teammates and opponents. It’s irrelevant, unless you can show that air travel jacked up offensive numbers (it didn’t.)
We’ve gone over this but I’ll try again. Please, please try reading this with an open mind.
The point to adjusting for era and park factors is that a player’s offensive numbers matter only in terms of how many wins they created for their team. That means that the number of runs being scored in the context a player scores in is immediately relevant to determining how valuable a player’s offensive statistics are.
In 1980, Mike Schmidt of the Phillies batted .286 with 48 homers and 121 RBI. In 2004, Adrian Beltre of the Dodgers batted .334 with 48 homers, 121 RBI. Who had the better years? Believe it or not, if you whittle the numbers down, Schmidt did. What Schmidt did in a league where the average team scored 4.03 runs a game was more valuable, put more wins on the board (not by much, but a little) than what Beltre did in a year when the average team scored 4.64 runs a game.
In 1968, Carl Yastrzemski of the Red Sox batted .301 with 23 homers, 74 RBI, and led the league with 119 walks. In 2000, Carlos Delgado of the Blue Jays hit .344 with 41 homers, 137 RBI, drew 123 walks, and hit 57 doubles to boot. Who had the better year?
In all likelihood, they were about the same in value offensively. (Yaz was a vastly better fielder.) Although his numbers were superfically inferior, Yastrzemski put them up in a league where very few runs were scored, and so his offensive exploits were proportionately more valuable. A home run in the AL in 1968 was more valuable, much likelier to win a game, than a home run in the AL in 2000. A single, a triple, a walk - all were more valuable in 1968.
The same can be done for pitchers. In 2003, Roy Halladay won the Cy Young Award with an ERA of 3.25. In 1968 that wouldn’t even have gotten him into the top 20. In 1968 Blue Moon Odom had a 2.45 ERA. Who was better, Odom in 1968, or Halladay in 2003? Halladay, easily. Halladay’s 3.25 ERA helped the Blue Jays far more than Odom’s 2.45 helped the A’s.
The same is true of park differences. Many home runs are hit in Coors Field. Very few homers are hit in Petco Park. A homer in Petco Park is likelier to help a team win a game than a homer in Coors Field, and so it is perfectly correct to adjust for that when determining which is more valuable - X homers in Petco Park, or Y homers in Coors Field.
The factors that matter in adjusting for context are the ones that matter in terms of determining how much a player helped his team WIN. Yes, a player today has coaching and conditioning Musial and Mays did not have access to, but who cares? When determining whether Yastrzemski’s 1968 was the equal of Delgado’s 2000, it doesn’t really matter WHY offense was higher in 2000 than it was in 1968. All that matters is that it was, so you have to take Delgado’s numbers with a grain of salt and understand that Yaz’s numbers are much more impressive than they seem at first glance.
We’re not talking about imaginary “what would have beens.” Park and era adjustments are trying to figure out WHAT ACTUALLY WAS. There is no comparison at all between a man who hits .300 with 15 homers in 1930 and a man who hits .300 with 15 homers in 1968. The former’s an average hitter at best, the latter is an MVP candidate.
On the whole, park and league factors pushed offense down a bit when Mays was playing versus when Musial was playing. They just did. Sportsman’s Park generally elevated offense; Candlestick generally depressed offense. I’ve pointed out before than the 1960s saw 10% less offense than the 1950s, which is a really big difference. You have to adjust for that stuff.