It's not about the skills required to make a t-shirt or app. Consumers don't care about that. All they care about is whether or not they're going to be using it. I'm usually hesitant to pay for apps because I don't know if I'm going to use them more than a couple of times. (Exception: I paid like $100 for Things 2 and 3, after using them during the free trial and realizing I loved them and was going to keep using them. And I have.)
It's easy to look at a t-shirt and think "Oh golly, I'm going to wear this all the time!"
Another really common reason people buy t-shirts is as gifts. I wonder if there's any corollary to that in the app world. Can people buy apps as gifts for friends? People will definitely spend ~$10 to send a friend a download link to gag app that mocks them, I bet.
A t-shirt is something you get, and can keep as long as you don't ruin it.
BTW, i recall reading that someone had more success with a "unlock feature X" model of payment, even though it in total was more expensive than unlocking everything at once.
If your customers were actually confident that your app really was worth $25 to them, they'd pay it. Unfortunately, it will probably cost you >$25 in marketing to get that across to them.
I suppose that if you sell a t-shirt via something like cafepress, you make something like $2 per shirt. If you sell a $3 app via app store, you make the same $2.
OTOH developing a nice elaborate picture for a t-shirt might be 20 hours, while developing a bare-bones first version of a useful app is 200 hours, and for a more polished app, easily 2000 hours over several versions.
So, a successful niche app should sell 10x to 100x copies than a successful niche t-shirt to bring in the same money.
However, as the recently featured Indie Hackers story about Scott's Cheap Flight shows, there are already a lot of possibilities to build businesses on top of productized processes and softwarem instead of doing custom work.
https://www.indiehackers.com/businesses/scotts-cheap-flights
In comparison I can buy t-shirts for 5$ so I would never pay 25 for one.