Because unlike in many other industries there are no strict regulations in software, there is no floor for the quality of the products.
So economic forces push quality to the lowest they can get away with.
It's much easier and cheaper to apologise to a customer for a bug and give them a refund than to ensure ahead of time that the bug doesn't happen in the first place.
The worst case scenario is not all that bad so generally businesses only bother to keep the quality high enough to compete but not "unnecessarily high".
If it costs $10k to make an app. You could also always spend $100k to make it more performant and robust. You could also go the other way and only spend $1k and have a slower buggier version. Where do you the line?
We end up reaching an equilibrium state.
Over the long term things get better incrementally. Individual vendors focus on their core objective and rely on those 'free' improvements.
For example a game maker whose game is a bit slow may make a 1 month effort to make their game faster and do what they can. But it's not practical for them to spend 1 year to achieve some sort of perfection state.