Vox senior producer Edward Vega explains why airplane seat pitch, colloquially known as legroom, has decreased over time. The simple explanation is money. Airlines have no legal legroom requirements, so it makes the most financial sense to offer it at a premium for those who can afford to pay.
In order to get the maximum number of people onboard, you have to either innovate — like by designing slimmer seats — or you have to shrink the seat pitch. Airlines have done both. Plus, they now give you the option to spend some more money to upgrade for more legroom, allowing you (and your wallet) to choose how comfortable you want to be.
Unfortunately, this works to a disadvantage for those who want to travel minimally.
If you want the cheapest flight, then you have to be willing to endure the slimmest seat pitch.