The USA, because of its size and remoteness from Europe and Asia, has relatively little international trade of goods (https://en.wikipedia.org/wiki/List_of_countries_by_trade-to-... has the USA sixth from the bottom) and didn’t, and, apparently, still doesn’t feel that pressure as much.
A major reason why the switch in the U.S. failed in the 70s was because the manufacturing sector revolted and didn't want to do the extra work to switch to metric.
I mean, what the hell, we are cutting their taxes to single digits anyway, might as well write this one off too.
The rest of the world has partially switched, but as long as people continue to use hours, days, weeks, months, years, leap years, and not kiloseconds and megaseconds, they don't seem to have switched completely either, but stopped at some point.
Even if you want to argue days and years are based on physical measurables, hours, weeks, and months are arbitrary and not based on 10.
Seconds (and thus all time units derived from them, e.g. hours, nanoseconds) are officially part of the metric system.
But yes, it is still worthwhile to keep in mind that despite nearly all countries officially adopting SI, a much lower percentage of humanity uses SI exclusively in their day-to-day life.
Seconds are indeed a fundamental unit of the metric system.
You can then use the standard multipliers and get things like kiloseconds and megaseconds, milliseconds and nanoseconds, some which are widely used, while some are not.
You the can also use legacy minutes and hours, which are based off the fundamental seconds unit, but that doesn't make them "metric", nor technically a SI unit (although it's accepted to be used alongside SI units. "Although not an SI unit, the minute is accepted for use with SI units" Source https://www.bipm.org/en/publications/si-brochure/)
The reason why you should do it is so that the rest of the world will understand what you are talking about, the cost is about 10 seconds with a calculator.
But the cost of making that switch maybe far outweighs any gains.