It is "a" problem, I'll even admit a major one, but it's not the main problem. The main problem is the expectation of continual customization and improvement at the input of the customer. Imagine if houses were built by you contracting with the developer, and after the initial blueprint and plans were agreed to, every two or 4 weeks you came back with requests to do new things like "add a basement bathroom", or "push out the back another 5 feet and add a glass skylight there for a sunroom", or "change the roof tiling to something 30% heavier".
Even without the regulatory hurdles and permits, nothing would get built as a sheer matter of physics. Blueprints and architecturally sound structures take some basic customer requests at the outset, and then make dozens of important, non-easily reversible technical decisions known to an expert to be required to support those requests. You can't just add on new customer-desired trappings without vastly compromising the quality of those assumptions for the original design.
But that is the situation in modern "agile" software development, and why selling it is so weird - because nothing else we'd say requires professional skill is built this way, and the real profit comes from annual recurring revenue - not a one time sale. Even wedding cakes have limits to change after the initial tiers are baked.