Because they don't want other people to know. There's no incentive for the company to share this information with you or anybody else. Why would they want your friend to know how much (or little) of the company your they are getting? They also don't want this information getting into the wild for the world to know.
But it DOES matter, and your friend should press for this. However, unless they are a particularly desirable or high-level candidate, they are unlikely to be successful.
Your friend might still choose to join the company even if they don't have all the information, but it's a bigger gamble and they need to know that they might not really have that much value in the options and consider the offer accordingly.
I've joined companies without knowing what fraction of the company my options represented and not regretted it. It is what it is.
Also, they should be very clear what the vesting schedule and triggers are. If they leave after 18 months, while the company is still private, do they keep any of the options? VERY important question! It is trendy these days (for tax reasons) for vesting to require both time AND a "liquidity event" (going public, getting acquired, etc). So your friend can get trapped where they have $1M in unvested options that they don't want to leave behind ("golden handcuffs"), but those options may NEVER vest.