This is probably the main reason in addition to it being less trouble over all when homebaking. One of my favorite things about morphine was the histamine release and a lot of people associate that itchy feeling you get with the product being good. Heard so many wives tales about one type of oxycodone pill being better than the other just because people would sometimes get lucky and get a histamine release despite the actual active being exactly the same (e.g. Combination tablets with Tylenol being better feeling that what they got from the old school OCs that had already had the oxycodone extracted (they weren't around to witness it)).
One of the things I regret the most about my opioid days was never getting to try pure 6-MAM by itself. I swear if I had the means I would make some. I almost did it once but I didn't want to risk wasting my limited supply of morphine capsules.
Outside of stability there is also the cost factor for the legal outlets. Only having 1cc being able to hold 60mg or so is much safer in a medical setting. Less risk of mistakes and they can charge more for the same amount of material (more solution=larger bags/bottles). I'm sure the whole preventing it from being diverted (or less desirable) for conversion to more fun is part of it too. In fact, if I bet you look around you may find the WHO requires it be in a sulphate solution.
I do know a bunch of different formulations of morphine were on the market and sold OTC wayyyy back in the day because my Great Grandfather was addicted to it back when it was still legal. He had a big collection of 'snake oil' bottles that contained various formulations of morphine and cocaine (sometimes in the same cure all). In time he moved on to buying from a pharmacist once he got further along in his addiction and I heard he stocked up when the laws were about to change. No idea how much he had laying around but I know he was forced to kick the habit sometime between WW1 and the middle of the Great Depression. I think he mostly kicked due to the economy going down the tubes though and not the inability to continue buying it. As a lot of pharmacists were still supplying by writing prescriptions for many decades after it was declared illegal.
He also bought a lot of miracle water from a local spring and paid big bucks to have it bottled and delivered to his house. He'd regularly ride his horse and wagon to the spring many miles away and later go by car once he bought one in the late 20s. He was pretty wealthy and into all sorts of things I probably shouldn't talk about here. The man hated Revenuers.