Why is the ability to drink salt water so rare in land-animals?
According to an answer here there is around 356,000 km of coastline on the planet. If we take an average human walking speed of say 5 km/h then our 5 minute walk takes us 0.417 km inland - giving approximately 150,000 km^2 of land conveniently supplied with sea water.

Now according to this page the total amount of land area is around 150,000,000 km^2 which according to this page is covered by somewhere in the vicinity of 117 million lakes [I'm certain a large portion of them are more accurately described as puddles but well go with that number for now], that gives on average a fresh water body every 1.3 km^2.

If we simplify things and imagine dividing the land area into 1.3 km^2 grid with ~1.14 km the distance separating each watering hole meaning the furthest distance to a watering hole across all the earths land area (ON AVERAGE) is around 0.570 km or just under 7 minutes walk (using our previous 5km/h)

So according to my very very rough calculations only around 0.1% of the land area is located within 5 minutes of sea water while 100% is located within 7 minutes of fresh water! I don't think that extra two minute walk is going to be that much of an evolutionary drive for any species.