Two jobs ago, I joined as the first ever tester to a team that had 20+ years of development without testers behind them. They hired me because they had figured out that when they log big visible error messages, almost 18 percent of use after login result in one and were puzzled on what the users do. It took me a year to get them to 0.1% but also teach the developers that they did know what was causing those, holding space pairing with them while they’d just look at me for confirming they would be doing the right thing - which they did not do without me reminding them at first. After 4 years at the company, I was still the only tester but the developers now knew how to test.
In this job I do now, with my previous team, I felt a bit of a deja vu going back to the idea that developers don’t test. They didn’t because they believed they were too valuable to pick up their own trash, and instead of sticking around garbage collecting, they are now again without a tester and management guidance on expecting things to be different. I had to step away to stop being their excuse.
Meanwhile, another team next door does brilliant without a tester and takes ideas from me on how they could improve their testing on our virtual hallways.
So my take is that teams are different and for many teams, having a tester makes them worse.