I don't think people realize how much it costs to fab
Some numbers for you... Bear in mind I'm an analog guy so I cannot speak to very small process nodes. For me to do a mixed signal design at 180nm, costs break out as follows:
Design NRE - $$$. Disciplines here are requirements development, requirements verification, functional decomposition, functional allocation, circuit synthesis (schematic capture), nonlinear circuit modeling, physical layout (chip artwork), further modeling. So for a simple HV op-amp you've got about a man year or two in before you talk to the fab. That's a quarter to half million, burdened rates. If the design is digital we would do an FPGA implementation first (*)
The cheapest way to fab is use a "shuttle run" where you team up with other vendors and split the cost of the mask between yourselves. A mask exposes perhaps 100mm x 100mm area; you might get 20mm x 20mm of this for your work, with a yield of perhaps 20-30 good die when it comes back. In one to two months. Cost at 180nm is around $25-50k. Faster? Pay more.
Now you have to saw, package, test. Typically the first mask or two is a no-go. Full functional plus HALT/HASS testing will consume another man year or so, and requires capital equipment. Call it an additional quarter of a million plus any additional mask sets - and this assumes the design is reasonably successful.
So that's why a simple circuit can push up to the million dollar level quickly, and timelines are long compared to, say, software innovation.
(*) Essentially any digital logic can be implemented in FPGA fabric. Microprocessor designs generally get prototyped and tested that way before a design goes beyond prototype phase There are numerous ARM and other"soft cores" you can license, tweak,and incorporate into your FPGA. One very interesting multicore microcontroller - the Parallax Propeller P8X32A - is wholly open source and you can use this in your own FPGA. See: https://www.parallax.com/microcontrollers
Downsides? Vastly more power hungry than custom silicon. Typically slower. Really expensive. Large. But of you insist your micro is the one true micro to rule them all, that's where you start.
Fun things to play with are the Xilinx CoolRunner CPLD and Cypress PSoC devices. A mere mortal can afford them, and quickly learn that doing custom digital is really, really hard.