Test suite is not free to create or maintain. To your example, for a product that isn't mature, a UI test suite is a waste of time and greatly increases the cost of UI changes.
The cost/benefit isn't there in the beginning. All the bugs are either obvious, easy to find, or quickly discovered without a test suite. So save the time writing tests to stamp out more features the customer wants!
Letting the users find the bugs costs time - you have to talk to the user, replicate the bug report, find the failure, fix it, and then release. Finding that same bug 2 minutes after you wrote it (and before it got released) is massively quicker.
This feels like one of those "write code 16 hours a day to make more progress!" things that experience has taught me just doesn't work in practice. I feel like I'm making a lot of progress, but after about 10 hours most of the code I write is pure shite and needs to be rewritten the next day. I've found that it's actually way faster to quit coding after 8-10 hours rather than spend 2 hours the next day rewriting everything I wrote in the last 6 hours the day before.
That's not realistic.
Your last paragraph about overwork seems unrelated to TDD dogma.
Ok but how confident are you that your business will work out?
1%? you just increased your project costs 100 fold by doing tests.
50%? you just doubled your project costs by doing tests.
100%? Ok you are saving money, but if you are that confident then why haven't you gotten investors aboard that are convinced that doing it well from the start is a business edge?