There are three kinds of cases where I've seen this information being hard to get:
1. Equity isn't a major part of compensation, and salaries are market-competitive. The employer says, in essence, "equity is not an important part of your compensation; let's concentrate on making sure you're happy with the salary and bonuses".
2. It's a relatively large company that is constantly hiring people, and they have a strong BATNA.
3. They simply don't know the answer, and the person hiring you isn't in a position to work with you to generate the answer (that work being "make three wild guesses at where revenue will be in 5 years, do a little research on what multiples companies are acquired for in their space, and figure out how much runway they have now"). If you know the % of the company you're getting, the dollars invested, and what the liquidation preferences are, you can come up with a somewhat decent guess here on your own.
If you're a key hire and equity is a significant part of your compensation, you should be able to get this information.
If equity is a significant part of your compensation, then, no matter what, you should be able to get:
* The number of basis points you're getting in the company
* The dollars invested in the company so far
* The company's revenue target for the year
* How much runway the company has at current headcount
If you can't get all of these, value equity at zero.