Once you get past one or two people building an MVP suddenly you need a project manager or a test team, or marketing, or finance team, or investors. All of a sudden things start to go bad.
If you're an enterprise company you decide to outsource to get a small job done and suddenly lots of extra people are added who are essentially overhead. It seemed cheap and now it isn't. Or you decide to buy a product to fix all your problems, but the business doesn't respect the limitations of the product and you end up buying another, or spend all your pennies on integration.
I've worked on project teams from 1-100 and generally the most productive ones have been with around 3-4 people.
Nowadays the all-singing, all-dancing one-stop-shop that has become the user's expectation has pages and pages of requirements, complex processes and fail conditions that could not only fill several books but are dependent on a whole host of other systems.
In short, it's user expectations (which ironically is the same reason it's more expensive to make a movie these days)
As time progresses technology will become more complex and powerful. Where the use of the technology may become simpler or the tools to produce make the work easier, the overall complexity of the software will increase as time continues. The amount of platform increases, the amount of users increases, and the amount of edge cases increases.
When you couple this solemn fact with the idea that inflation has increased what would have been tens of thousands of dollars of development costs in the 50s to the millions that exist now - the cost has increases as everything has, and the complexity of understand and designing the system has increased more alongside it.
If you consider that there are more developers than ever before who are all competing on salary then the price of employment increases as well.
There are many reasons for your question, and really it all comes back to the initial line.
The costs to develop software are ridiculous and unrealistic. It turns out that if enough people act as ridiculous and unrealistic that becomes the new normal.
Personal and sales websites are cheap now. Blogs are cheap. Web infrastructure is cheap.
Social media is basically an information superhighway. Compared to actual highways they take more traffic and cost much less. Many other apps are replacing middle men; Uber is quite cheap for something that is replacing the whole taxi industry.
The tools, libraries, frameworks etc. we have now make it a lot easier to build something that would have earlier cost a fortune in custom development. If anything, it's getting cheaper to build projects now.
Even with the huge financial incentives supply of developers is constrained by the mental capacity/desire to do the often time consuming and tedious task of learning the development skills.