SELV is often capped at 60 V, not 50 V, although this is often treated as “48 V with crappy bounds”. An example that takes advantage of this would be PoE, which allows 44 V - 57 V at the PSE; but most PSEs are, in my experience, in the 54 - 57 V range.
Only for 48 V systems that have a battery, which does not apply to PoE, and does not apply universally to 48 V automotive approaches (although the CyberTruck does use a 48 V battery).
We're talking about the LV rail (nominally 48 V or 12 V) in an electric car. There are two design paths here -- you can have an HV to LV converter capable of supplying the average load, and have an LV battery; or you can have an HV to LV converter capable of supplying the peak load, and save the cost, weight, and failure rate of an LV battery. Tesla chose the former, but it's not the only path here.
Sure, in an EV it's possible to choose not to have that second battery. In an ICE car, it's needed to start the engine. So at least the ICEs are much more likely to settle for 48V than something closer to 60V, to leave room for charging. The manufacturers that make both ICEs and EVs are going to want to share components, so the pure-EV components are likely going to be built for 48V too.
There are massively diminishing returns. 1kW at 12V is 83A, which needs a rather beefy wire that is expensive to buy, annoying to route, difficult to route, and needs careful termination with large tools. 1kW at 48V is ~20A, which can be done with 12 gauge wire, which is not bad at all to work with.
