Because tzdata is based off of political boundaries and decisions. If I'm making a dataset that needs continuous timestamps for multiple decades then UTC is effectively off the table because of how under-specified and poorly implemented leap seconds are in various software. So what's left is TAI and GPS, fine enough but now users see the timestamped data as being several seconds off. So we use a translation layer but some HMIs don't allow that so it's best to just not do it at all.

This reality is not unique. Thousands of engineers have walked down this path and asked "why?".