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 $2.
OTOH developing a nice elaborate picture for a t-shirt might be 20 hours, while developing a bare-bones first version of a useful app is 200 hours, and for a more polished app, easily 2000 hours over several versions.
So, a successful niche app should sell 10x to 100x copies than a successful niche t-shirt to bring in the same money.
However, as the recently featured Indie Hackers story about Scott's Cheap Flight shows, there are already a lot of possibilities to build businesses on top of productized processes and softwarem instead of doing custom work.
In comparison I can buy t-shirts for 5$ so I would never pay 25 for one.
Are they reliable? Do they pay in time and for all the sales? Do they hide sales?
I would argue that now is a better time than ever to be a musician given that with the internet, a record label is not strictly necessary and you can market and sell merch to a massive audience and make money with just an internet connection.
In the last 10-15 years maybe, but up until the nineties or so, merch (even t-shirts) wasn't much a thing for most indie musicians -- except in the metal world perhaps.
As for tours, they were seen as loss-leaders for album sales. Some select concerts in big cities could get money, but the expenses of touring around e.g. the US where big too, and in the end it was a bet in getting even.
On the other hand, in the vinyl era, indie musicians (which had better profit-sharing deals and recorded cheaply, not spending $200,000 for farting around in the studio for 6 months) could sell 30.000 - 50.000 albums or so and be able pay the rent from that.
I sometimes feel a bit cynical about it, but I'm glad they found a way to fund their tours & keep churning out more of my favorite albums every year.
Edit: To be closer to topic, Panic  used to sell t-shirts & hoodies, Atebits  used to sell pillows of the Tweetie app icon, and The Iconfactory  sold models of the Twitterrific Ollie blue bird.
Make of that what you will.
But in custom t-shirt platforms we have many competitors.
So why isn't there one winner ? What's different ?