Earlier quoted context omitted.
Because this is ISO 8601 date format, the standard date format that intentionally uses dashes to not confuse 25/01/22, 01/22/25 and 22/01/26 which are the same date in different formats (that's why it uses dashes for dates, as for why it uses / for interval I have no idea)
It's using a slash because that's how ISO 8601 defines time intervals. https://en.wikipedia.org/wiki/ISO_8601#Time_intervals
RFC3339 at least lets you replace the ugly T with a space but also doesn't specify any spaces around "/" for ranges.