But engine blocks and truck frames & bodies work for grounds...
Not saying you're wrong, but it also doesn't quite make sense.
I can understand how this is hard to wrap your head around. The most experience with this problem is in the boating community where stainless is common. You will find lots of forum threads on the topic with Nigel Calder, a legend in boat maintenance leading the cry to get this word out.
Here are some differing proposals. Maybe one will make more sense.
Anytime something is massive enough, it will conduct well enough as in the case of an engine block or frame....relative to a washer it can be said another way, anytime something is resistive enough, it needs to be massive.
I searched out a more complete list of metal resistivity specifically so I could find 304SS which is what most common stainless hardware is made from (similar to 18/8 which is also commonly used).
A list of the conductivity of metals sorted by resistivity from silver to graphite.
eddy-current.com
The conductivity of 304SS is 2.5% that of copper (see table link). High alloy steel is barely better at 2.9% and iron is 15-18%.
If you want to use 304 in the same way you use copper, it needs to have 40x the cross-section area or 6.3x the diameter.
While the 'length of wire' in a washer is very short, that fact works against it for heat dissipation. Current experiences all that resistance/heat in a small space. Even if it were in free air the concentration of heat generation and small surface to radiate, results in heat flow (conductive) into the wire or fuse. If they are on a fuse lug, the heat created by the washer significantly reduces the fuse ampacity (because fuses function by resistive heating and the terminal heat adds to the fuse material heating).
Fuse materials are aluminum/tin or aluminum/zinc alloys with a relative conductivity in the 40-50% range.
Using SS/steel bolts, nuts AND WASHERS is just fine as long as they are not in the current path. If they are only a clamp, they are so resistive, almost no current will flow through them. Placing a washer between two lugs, forces the current to choose between the high resistance path through the washer or the high resistance path through the bolt, since the washer is thinner, it gets most of the current.
I fully understand if you still can't believe it. All I'm advocating is to be sure you don't put a SS washer between cable lugs or on the bottom of a terminal post. If you just give me the benefit of the doubt, you'll never have a problem.
