It's about case and effect: seat costs for specialised tools are high because they can be. People with limited B2B experience underestimate just how much cash large corporates can throw around without even noticing, and how rarely ROI is assessed on this spending.
The development costs required to create a competitor to an industry standard product can be relatively small. (Sometimes they aren't - but they can be.) The real costs are in marketing and sales.
Quicken doesn't do anything outstandingly clever, but good luck trying to sell your new competitor to existing accountants.
Generally there's only a change if the incumbent gets too greedy and complacent, or if the industry changes significantly and a new generation of companies appears with new ideas about which software is "standard."
It seems like nearly every profession has high-priced software that's required: artists, engineers, accountants, lawyers, doctors/nurses, research scientists, architects, etc, are buying products from Adobe, Quicken, West Law, Reed Elsevier, Epic. None of those are small companies, yet all of them focus in somewhat niche information technology sectors. I'm struggling to come up with a profession that doesn't touch software anymore; even retail workers use point of sale and inventory software.
I do agree with your sentiment though. The software is expensive because it's so niche. The market for all of this stuff is pretty limited, meaning there's only a few products offered, and the capital required to build competitive products makes it prohibitive for a company to build their own. The companies making the software also understand their target industries intimately enough to know exactly what price the market will bare.
However, it does leave an interesting market opportunity for and engineer + domain expert to tap into the market with cheap, low-feature offerings. Or maybe we will see more companies bringing development back in-house.