Another way to think about it is that UTC is a combination of UT1 and TAI. It’s kept within 1 second of UT1 and is always a fixed integer offset from TAI.
TAI is an absolute time based on atomic vibrations. UT1 is a relative time based on astrometric systems.
It's not a fixed offset from TAI. The offset changes every time there's a leap second.
Also, there is no such thing as absolute time, because of relativity. We take pains to account for this by only measuring TAI on the Earth's geoid, but even so the uncertainty never reaches zero. Two "stationary" atomic clocks on the geoid are expected to drift by about a second every billion years or so. Sure, that's pretty good, but definitely no "absolute".
When you really think about it, the cesium atom and the earth's day/night cycle are both equally valid clocks. But the cesium atom measures time more similarly to the way we perceive time on a small scale. But, of course, our lives are scheduled based on the day/night cycle. So they both matter. UTC is the way we reconcile the two.
TAI is an absolute time based on atomic vibrations. UT1 is a relative time based on astrometric systems.