They are. The second is the only unit of time in the metric system. All of the regular SI prefixes apply. They just don’t fit the earth day perfectly, and nothing will, nor should they. Being earth centric is not good science.

There are benefits to numbers other than powers of 10 that decimal doesn’t really give us.

A lot of measurements we deal with are based on 12 (hours on a 12-hour clock, months in the year, inches in a foot), 24 (hours in a day), 60 (minutes in an hour, seconds in a minute), 360 (Degrees in a full circle, seconds in an hour, and at one point, the number of days in a year, before we started to realize that after enough years passed, the month you used to think of as winter started being warmer.)

And one thing all of those have in common is divisibility by 3.

1000 is divisible by 2 and by 5, but not by any other prime number.

12 is divisible by 2 and 3, and all the other examples I gave above are multiples of 12, so they’re also divisible.

Dealing with half of something is pretty straightforward. Dealing with a third of something, also not difficult to visualize. But a fifth of something requires a little more cognitive load. (Think about trying to evenly divide a pie in 3rds vs in 5ths.) Even a 6th is probably easier to deal with than a 5th, and that’s largely because it 6’s prime factors are smaller than 5.

With the metric system, everything’s divisible by 2 and 5. 100’s prime factors are 2, 2, 5, and 5. 1000’s are 2, 2, 2, 5, 5, and 5. A third of a metre is approximately (but not exactly) 333.33 millimeters. A third of a foot is exactly 4 inches. The metric system also doesn’t make it easy to evenly divide by 6.

We could make a whole system of measurements based on powers of 12, I suppose. But our dominant number system has been based on powers of 10 for a long time.

Maybe even better would be to use powers of 30 because it’s divisible by 2, 3, and, 5 (and 6). But it’s not evenly divisible by 4! So maybe one based on 60 which is divisible by 2, 3, 4, 5, and 6.

Well, now we’ve ended up with a big part of the reason why there are 60 seconds in a minute and 60 minutes in an hour. In fact, historically, there were cultures which used base 60 as their number system. For instance Babylonian mathematics.

Maybe the really best way to go about fixing our various ways of measuring things is to abandon base 10 in favor of base 60 and then invent a “sexagesimal metric” system.