Because you have to steal at a very high percentage to break even in adding runs created to your team.
In today's run scoring environment where the home run is so important you need to score at close to an 80% success rate in order to even break even.
Lets say a modern player goes crazy and steals 140 bases in a season at a 82% success rate.
All of that running make look impressive but they would be adding just over 5 runs created to their offensive total.
Stealing bases just does not get you a lot unless you can do it at such a high rate that your almost guaranteed to never be thrown out.
Here is a good article from 2011 for various stealing scenarios.
https://blogs.fangraphs.com/breaking...k-even-points/
In the 8 years since the %s have climbed to 80% for almost every situation.
Giving away outs is just not a very smart move in modern offenses.
It is why the smarter teams do not do things like sacrifice bunt or other such nonsense. You only get 27 outs per game and they are way too valuable to give up.
Even more so when all nine guys in your lineup are capable of hitting 15 HRs a year.
The three true outcomes style of modern offense may be boring to watch but smart people in front offices have figured out the most efficient ways of scoring runs.
A strikeout is the same kind of out as a popout or a groundout to second. So might as well swing hard and increase your chances at a EBH.
And it is drilled into every batter from HS level on up to work the count and try and get on base via BB. Many still cant do it but more can do it now than in the past.
Once you get on, even the fastest players add very little via base stealing and most actually hurt their teams run creation chances by getting picked off or thrown out.