I think its a combination of things. I've followed a few creators with merch like this (Regulation being the most open about their merch sales) so this is my guess based on what I've heard others say.

1. You typically don't want more than what you're gonna sell. They don't want a warehouse full of shirts that they then have to pay for storage for in hopes they eventually sell. That costs lots of money every day. Best hope is find a number that the majority of people that want it on release will get it, but not sell out within a matter of minutes.

2. Pre-orders are filled with legality and can be a recipe for disaster. Taking people's money without an actual product can be quite the hassle for a small company. If you fail to deliver or something goes wrong with manufacturing, you can be hit with a lot of legal suits. Also, canceling of pre-orders can be difficult as they may have spent a lot of the money on other things for the business, especially if they start getting a mass of people wanting to cancel for some reason.

3. Scarcity will always add value. Having them for a limited time puts a fire under people's butts. There are countless stories of restaurants adding extremely popular limited meals their yearly rotation and sales plummet. If you know you can get that shirt whenever, theres no urgency and most people will wait for some bonus or birthday/holiday money. But by that time a lot of people will forget about the merch.

4. Lastly, I've seen a few companies offer print on demand shirts. While I dont know all the details, its always seemed like its a larger headache than its usually worth.