Because there are loads of medical people still saying the same now and if, for example, they were a raging success why would cases soar in a place like Scotland where they were enforced? If something is proved as inconclusive surely you make up your own mind?
It's a good question and I'll try to give part of the answer.
Firstly, there are masks and then there are masks. They range from basic materials, through surgical masks, then FFP2/N95/KN95 masks through to FFP3 masks. You could go on to full bio suits and respirators as well. The better, the quality of the mask, the better the theoretical protection. I say theoretical because a mask is only going to perform to its maximum protection if worn correctly as well.
Then we need to consider where and when the masks are worn/not worn. Somehow, I doubt anyone wears a high grade mask 24 hours a day, so the situations where people don't wear a mask also need to be looked at. A chain is only as strong as its weakest link.
As I've said before masks are only part of the answer, but a high grade mask, worn correctly, will offer a very high level of protection whilst it is being worn. Wearing a mask in some high risk environments, but not in other high risks environments, will seriously compromise your overall protection.
Even when masks were mandatory in indoor environments, it was fascinating to observe how many people thought that their mask was a fashion accessory rather than some form of protection; mostly poor quality masks that for many people I saw, seemed to be offering what little protection they would afford, to their necks.