Because they spent that much money on a CPU only to now have to buy a new board to take the next (small) step.
Sick of Intel changing boards all the damn time. Two years may seem like a long time, but what can they upgrade to? oh yeah, nothing.
In fact, the only difference on 490 is Wifi 6 support. Why on earth would you want a new board for that? Especially given Roman pointed out quite clearly today that 390 boards could have easily ran these CPUs. Why not launch these CPUs now on 390 instead of all of the "hoo har" and so forth and then when PCIE 4 might actually work release those CPUs on a new platform?
You know what? I don't care how good these CPUs are. I've learned my lesson very stiffly from Intel and their constant sodding socket changing. All it means is if you have a board failure your replacement options are used boards only, and they quickly become very expensive because people know you are screwed without them.
On AMD? hell, I could go and buy a cheap B450 board and be back up and running in no time.
Then let's not forget price. My X570 TUF 12 phase was £185. Not only does it actually support PCIE gen 4 but it also supports a 16 core CPU *and* overclocking that CPU. Let's see what these 490 boards cost huh?
I read a study lately that said in many cases manus make little to nothing from Intel boards. They literally just do it to stay current and big up their branding. Some AMD boards have been that way too, but only at the super high end. I would be very interested to get the figures on what Intel charges for chipsets and licensing VS AMD.