If you try to talk yourself through how you'd actually accomplish that then you'd quickly see that it is turtles all the way down. Every time you discipline the clock for accuracy you trade some precision to do so - because that is the only way to move the needle. This isn't a big deal for most, but you'd definitely notice NTP slewing the oscillator frequency in the middle of something like a logic trace running at 10s of MHz - which is why labs generally prefer a precise local time standard that is costly (in dollars and hassle) over a cheaper GPS reliant solution offering better absolute accuracy.