It's of course a trade-off in how far you want to go with special types.

Booleans and numbers have extremely common use cases, moreso than date/times.

But perhaps more importantly, they are quite easy to define. Date/time is a susprisingly complex topic with many variants (date, time, datetime, local/relative date/time, point in time, offset-based, timezone-based...) with all of them being quite important. The spec to define date/time types would likely be longer than for the whole rest of JSON, and you still wouldn't be able to correctly interpret the date/time from the spec alone, since timezone data/designations are dynamic.

Now the question is - what value does this extra complexity bring? I'm not saying there isn't any, but it doesn't seem to justify the cost.