TPMS essentially automates people checking their tires because the reality is most people do not do a walk around on their car before driving.

In theory, we could use a dipstick in our fuel tank but most of us prefer an automated gage.

Even if you do a walk around, under-inflated tires are typically not distinguishable from normally inflated tires. Especially on today's cars with shorter and stiffer sidewalls.

I had a rental Mercedes with a leak in a tire recently... a tire was at something like 15psi but looked visually the same as the other tires. I absolutely do a walk around on all of my rentals and take pictures, but I would have had no clue if it weren't for TPMS. I would have driven it until it failed.

It had already failed. You had run-flats on the car. They’re still visually apparent.

Tires can have low enough pressure to affect vehicle handling without being visually low, you simply cannot measure tire pressure visually. That's why even tire shop workers use a gauge instead of eyeballing it.