It's a bit more complicated than that.
You have to consider peak voltage if you're talking contact ratings.
So say you have 1KW into a 50 ohm system, that's 223 volts RMS (P=V^2/R) but it is actually around 624 volts peak to peak (Vpp = 2 * sqrt(2) * Vrms)
Now assuming a crappy SWR, assume that reflected power is all coming back, that means on a bad day your contacts are handling 1248 volts. 2.5KV isolation isn't uncommon in RF relays. You have a much bigger problem if the SWR is that bad because your little output transistors will say bye bye in record time. Reverse power protection is required.
Now imagine several tens of kW and you then have vacuum relays and all sorts of stuff.
This is one reason my TX is a more manageable 10W (still need a license before I can use it on air
) as all the protection is, is a fat zener (which brings other miseries but not those of blown PA transistors). Perhaps one day I'll build a big PA for it.