Because it may be really *expensive*.
I believe in this case there's no free lunch. I think if a user can't accept a couple of minutes downtime for a reboot, but isn't willing to invest in spare hardware won't really get high availability, only a sense of high availabilty.
I've checked CVE and found 10 linux kernel problems for this year. At this rate there could be 30 such problems in a year - if all problems need a reboot, than that's about 60-90 minutes of downtime. I guess a hardware failure would lead to a longer downtime (it takes time to get the new hardware) and a hardware failure isn't schedulable to off-business or off-peak hours (unlike the kernel change).
To me, this solution looks to be more like a toy (or a weird hack, as it was put earlier in this thread). It's nice, but I wouldn't use it for real.
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