Why don't gas water heaters have leveling feet?

I just wanted to say throw a shim under the legs that need it, the floor isn’t that crooked 

They dont have to be level to operate properly. If you floor runs down hill that Bad, I would place a shim of some kind to level it up to some extent. 

Why SHOULD they have leveling feet?

The assumption is that you are going to install the heater in a home, where the floor is already level.  Leveling feet are not necessary.

If the floor is not level, it's usually close enough that adding adjustable feet is still unnecessary.

The flat base of the heater spreads its weight out pretty well; feet, on the other hand, concentrate the weight of the water heater onto however many points the feet represent.  You go from less than 1.4 lbs/sq. in. on the floor to (assuming four feet, 3" dia. each) to over 20 lbs/sq. in.  That may not sound like much but don't forget, the water heater sits in one spot and never ever moves.  Eventually, it's going to sink into a wooden or vinyl floor if that weight is concentrated by feet.

A floor sloped toward a drain is indeed on a slope, but just how steep a slope are we talking about?  The few times I've felt a water heater needed to be leveled up, I did it by shimming underneath with a few layers of roofing felt.  Worked great, but it was barely worth even the five minutes it took to do.  It didn't add anything useful to the heater at all, it was solely for looks.

"What kind of stand," good grief.  ANY kind of stand.  In thirty minutes I can bang together a stand that will hold up both the water heater and the pickup truck you brought it home in, that isn't hard or even time consuming to do.  How weak do you think building materials are?  A single 2x4 has sufficient strength to hold up FOUR full 60 gallon water heaters, the only thing requiring any kind of thought is making sure the stand is sufficiently rigid that it doesn't fold over sideways.  And that's so easy it's barely worth talking about.  And again you come back to the issue of concentrating the weight of the heater onto the stand's feet.  If you don't need the stand, don't bother.

NOTE: California Plumbing Code requires getting a burner at least 18" above the floor unless the water heater has been manufactured with Flammable Vapor Ignition Resistant Technology, which has been required of water heaters of 30-50 gallons since 2003.  So the stand is required by 2010 CPC 508.14 but is obviated by the heater itself.  If you have an older heater you need the stand; new ones shouldn't unless they fall outside of that size range and aren't provided with the factory safeguards.

I would not buy a house that had a floor drain in the basement because if water can go down, water can come up. {Any slope would be very gentle.} Besides, there should not be any water leakage anywhere anyway.  A ghw tank can only be put in a basement if there is easy access to place the tank in the basement, and the exhaust chimney has an easy way up and out.  My ghw tank is in my pantry, and no stand underneath it.

Short answer (imo): the tank would be vulnerable with all of the water weight pressing down on the stress points of the leveling legs.

Short answer: They DO.

mermeliz can't cite the relevant code, and wouldn't recognize it if someone quoted it.Note: My gas water heater was installed directly on the floor within the last 2 years. My gas furnace was installed directly on the floor last January. The city inspector didn't say ONE WORD about a stand for either. Raising the flame a foot or two wouldn't help in case of a gas leak anyway, as the gas lines are nearly always ABOVE the flame.