Is it? I've never heard of anyone who wanted to try drugs and was unable to get them. Maybe it's different in the US, but in Europe, you can always find a supplier.
But I also know that I would be risking various legal and social consequences. A conviction for possession of drugs would affect my employment options, it would affect my travel options (visa forms often ask about such convictions), it may cause issues in the future if I ever got into a custody battle for my kids.
Given all of that, I have no plans to walk down the street and buy some weed from my friends' kids.
But I do have a collection of cigars in my house which I enjoy on occasion.
If it were possible to buy marijuana in the same way I can buy cigars, with the same legality, for a comparable price, then I might do so.
Now, I don't really see why society should care if I were to substitute some of my tobacco consumption with marijuana consumption, but there's a reasonable probability that this would be one of the outcomes if marijuana were totally legalised.
And on the subject of drug availability, I'm aware of a few remote Alaskan towns where it's difficult to obtain drugs. I'm sure that there is some more developed and less isolated region of the world where it is also difficult to get such things, but that's probably not the common case. There is a global demand for such things, and there's only so much you can do to restrict the supply. Regardless, people will still find ways to get high. In the Alaskan towns I mention, bored teens will 'huff' paint fumes instead. There's not really a winning situation there; we can only minimize harm.