Why do switches say 10/100/1000?

Solution 1:

Not the same thing at all. Indeed, there exists no such thing as "up to".

10/100 Mbit/s uses the same cable (ignoring different specifications for the shielding, which is negligible from the switch's point of view). They use a different clock and might (I'm not 100% sure, don't nail me down on that one) use a slightly different signal coding.

GbE uses a much different cable (with two more copper wires) and most certainly uses an entirely different signal coding that pushes through more bits per clock. The clock rate is -- surprisingly to the unsuspecting user -- indeed not ten times higher than that of 100Mbit/s, only four times.
So, that's in principle a totally different, hardware incompatible thing, which only happens to have the same general marketing name "Ethernet", and the same kind of RJ45 plug.

The inventors of Ethernet were intelligent enough to add a very extensive negotiation capability, so it is possible (but not granted) that a device built for one standard also supports another.

Thus, while in practice a switch supporting GbE always (always? well, maybe...) supports 100 Mbit/s and 10 Mbit/s as well, it does not need to do that. It's a bonus, if you want to see it like that, and it is not "everything up to", but it's implementing three very specific, different, well-defined standards.

Solution 2:

They are entirely different standards - it's not like an ADSL connection that essentially does "the best it can" to get data to you at your hoped-for speed [the famous "up to" advertising lies claims made by ISPs until recent legislation to stop them] .

Each standard has a specific interaction. If you don't have the right cable, or the wiring is sub-optimal, the system will automatically switch down to a slower but more robust connection mode.

More than you'd ever need to know at Wikipedia - Ethernet over twisted pair