Software accessibility is bad for business - that's why it has to be legally mandated. There's no firm difference between "accessible", particularly to screen readers, and "amenable to end-user automation", and the latter is anathema to businesses.

As for the styling and fonts - it's all a bunch of low-key trade-offs here. The businesses want to exercise total control over the end-user experience, for sales/marketing and branding reasons. At the same time, going off the beaten path quickly makes development costs skyrocket.

Flash was a good choice back when all visitors were using a PC and there were like 3 different screen resolutions to choose from. Once laptops gained popularity, this started to shift, then pre-iPhone mobile briefly became consideration, and Flash stopped being sufficient; then iPhone came out, killed Flash, and started an era where every visitor has a different screen than the previous one. "Responsible web" became the cheapest option, and you can see its evolution as a trajectory towards giving more and more control over experience to developers, and less and less to the users.