Probably because if you’re joining an early stage company you’re inevitably going to get diluted if there are any later rounds. By how much will depend on how the company is performing so they won’t be able to know exactly what this will look like ahead of time. Perhaps putting the percentage in writing opens up some liability down the line as a result.

You should be able to get the number of outstanding shares at the time you join in writing though and then the percentage is trivial to calculate (though as other commenters pointed out you might not be able to get information on any existing liquidation presences investors might have that make it hard to really know what you would get if the company were to sell for x amount...