Those teams (Leafs, Oilers) have an offensive system with zero care for defence and even if you put prime Hasek in nets, he wouldn't be able to be as effective as you think.
If it's the hardest how comes you can pick a decent #1B off waivers? Picking a 2C or a 3-4th D-man ogf waivers is pretty much impossible.
It's not just two teams that are looking for goalies, and even if those two have more problems then just bad goaltending, they very much are getting bad goaltending and have tried to get NHL level goaltending unsuccessfully for years.
As for why you can get a decent 1B off waivers, it's the same reason you can get 100+ point scorers outside the top picks. Player evaluation is actually hard, Florida went with Knight instead of Montembeault and that wasn't because it's easy to get a decent goalie it's because they mis-evaluated the two.
In fact the reason goalies generally don't get much in trades, and can be found in later rounds or on waivers more frequently then other positions is because they are harder to evaluate. Even more established goalies with NHL track records can go from looking like a good starters to not even NHL level from one season to another, so teams aren't going to want to pay a high trade price since to an extent they are getting a mystery box.
So getting a 1B off waivers isn't evidence that it's easy to find/fill the goalie position, it's evidence that it's actually very hard.