simple example... why are only later days cheaper on a ticket, rather than simply lowering the day price across the board? Because the model is intended to make the stretch days of a vacation less of a commitment to pick Disney vs an alternative... or extend the stay at Disney 'because its so cheap' when someone compares the $30 a day vs the $100 a day they know the single day ticket is.
Why not take the same discount and as long as I paid up front.. let me use those days as I want? Because the pricing discount is not a discount for buying in bulk or for a larger transaction... it's a discount intended to stretch stays.. so the discounted tickets expire to ensure the discount works per trip, not for bulk buying. (and why Disney caps how long a MYW ticket can be bought for)
Using a simple argument you say "Disney is offering cheaper tickets.. which is what people want". Disney isn't offering cheaper tickets, it's offering an incentive to extend the length of your stay by offering discounted days...
When a guest compares going to Seaworld.. they now face 'giving up Disney discounts', and will compare paying Disney's $30/day rate instead of the regular $100/day rate. You can see how this makes it difficult for non-Disney properties to compete against Disney's super-discounted day prices.
The market used to survive by 'go to disney, and come see us on the side'. People would (happily) stay offsite, visit DIsney, visit Seaworld, visit the various Orlando town sideshow attractions, etc. People would day trip into Disney a few days and have a varied Orlando vacation. Disney has snuffed most of that ecosystem purely by how their products are structured and leverage each other. Not necessarily because customers suddenly ONLY care about Disney. But a decade plus later... people almost forget what the most common way to visit Disney used to be because Disney created new products and 'walls' through offerings to capture that type of visitor, then strangle out their ability and willingness to go outside Disney.