One problem: unless you have an endless amount of time, money, patience, and...did I mention patience? that just isn't a viable approach for anything but the smallest of hobby projects
Dependencies will complicate system you are building. ( with things like supply chain attacks, bloat, liscensing etc...)
There's a reason libraries exist. It's not like they were the default state of computing and no one has tried to write applications without them. On the contrary, we tried to build applications by writing everything ourselves, but that doesn't survive encounters with the real world. And "well if you can't do it yourself, you don't need it" (which may or may not be your argument, I genuinely can't tell) is just technologically regressive ideology.
Look at Big tech, this is what they are doing, they employ thousands workers that write millions of lines of code every day, only to make it work for every case in a world. And still can't compete with specialized solution.
Yes, I accept this is true, but your earlier claim was much more specific: that using dependencies at all makes things worse. I gave you a specific example (GUI libraries) but you completely ignored it. How does your 0-dependencies theory survive an encounter with that basic example?
That said, most sw projects are determined by business need. I can't think of a single one I've ever written that hasn't needed dependencies or would have been viable had I not used some. Going to a theory level, there is no reason to think the stdlib is magically immune to the issues of a very popular dependency either. Shun absolutes and make the correct trade offs based on your business goals.