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.