If your first party tooling contains all the functionality you typically need, it's possible you can be productive with zero 3rd party dependencies. In practice you will tend to have a few, but you won't be vendoring out critical things like HTTP, TCP, JSON, string sanitation, cryptography. These are beacons for attackers. Everything depends on this stuff so the motivation for attacking these common surfaces is high.
I can literally count on one hand the number of 3rd party dependencies I've used in the last year. Dapper is the only regular thing I can come up with. Sometimes ScottPlot. Both of my SQL providers (MSSQL and SQLite) are first party as well. This is a major reason why they're the only sql providers I use.
Maybe I am just so traumatized from compliance and auditing in regulated software business, but this feels like a happier way to build software too. My tools tend to stay right where I left them the previous day. I don't have to worry about my hammer or screw drivers stealing all my bitcoin in the middle of the night.