Money and resources.
Let's not pretend like we can't do it. We can, it's just not worth it. The issue is COVID-19 isn't that deadly and ventilators don't guarantee an improved outcome. The Americans made a CO2 scrubber out of duct tape and spare parts to get Apollo 13 home. The Russians found ways to toaster factories into AK-47 assembly lines.
If the fatality rate was 90% without a ventilator, and everyone was guaranteed to get it and need one, we'd be making them so fast in every way possible. 3D printed parts, CNC parts, stamped parts. What would normally take months could be reduced to days. Digital, mechanical, you could probably even make one that runs on an air compressor. Hell, if you could use ventilators as a weapon we'd find a way to make them in hours.
That's not to say it's easy. It would be really fucking hard. But it is possible. It's just not "worth" it.