A baseball season contains far more games than seasons in comparable sports. The length follows from what the sport demands physically and from how clubs earn money.
The physical load per game is low
Most players are stationary for much of a game. Bursts of effort are short and separated by long pauses, so recovery demands are modest compared with continuous-running sports.
That makes daily play feasible for position players in a way it would not be for athletes in sports requiring sustained high-intensity movement.
The exception is pitching, which places extreme repeated load on one arm and cannot be performed daily by the same individual.
Rotations solve the pitching constraint
Teams carry several starting pitchers who appear in turn, each resting for days between appearances while the team continues playing.
Relief pitchers cover the remaining innings, with usage tracked closely so no arm is used on too many consecutive days.
The rotation is what makes a long season possible. Without it the position-player workload would be sustainable but there would be nobody available to pitch.
Sample size makes the season meaningful
Individual baseball games are noisy. A weaker team beats a stronger one often enough that a short season would frequently produce a misleading table.
A long schedule lets underlying quality separate from chance, so the standings at the end describe something more stable than they would after a handful of games.
This is also why baseball statistics are treated with unusual seriousness. The volume of observations supports analysis that shorter seasons cannot.
Home dates carry the revenue
Clubs earn from admissions, concessions and parking on every home date, so the number of dates is directly connected to the money available to operate.
Stadiums are expensive to build and maintain, and a venue used only a handful of times a year is far harder to justify financially.
The scheduling structure of series played over consecutive days also reduces travel cost relative to playing the same number of games spread further apart.
The calendar constrains the total
The season must fit between spring weather and the point at which late-autumn conditions make outdoor play impractical in northern cities.
That window, combined with the need for occasional rest days and the postseason that follows, sets a practical ceiling on how many games can be scheduled.
Arguments about lengthening or shortening the season are therefore arguments about which of these constraints should give way first.