Um, because it's the prime meridian and that's how UTC is defined?

> It's nearly impossible to have your local time match your local solar noon.

Which is why I specified on the prime meridian, which is the particular local meridian that UTC is defined as corresponding to.

> solar noon varies from day to day by 10-20 seconds.

Which is why I was careful to specify mean solar noon.

I'm not quite sure what your issue is. Yes, we have time zones tied to specific meridians, and the actual sun's speed in the sky varies (which I mentioned in my post, so I'm not sure why you seem to think I'm unaware of it) so in most places local time by the clock doesn't match local time by the sun. Yes, a leap second adjustment to UTC is quite a bit smaller, taken in isolation, than the annual variation in actual solar time vs. mean solar time.

But over time, if we didn't have leap seconds, the difference would accumulate. The accumulated difference now between UTC and TAI is 37 seconds--which is almost twice the maximum variation in actual solar noon from mean solar noon that you refer to. We humans have collectively decided that we don't want that, and that it's better to do the adjustments a little at a time rather than in bigger lumps.