Nothing you're missing. 86400 is just the biggest expiry rate you can provide. If you provide anything higher than that, it gets set to 86400. This happens here:
On Kaltura's SaaS,
myPartnerUtils::getExpiry ( $partner_id );
will always be 86400. If you're running on CE or OnPrem, you can change the limit to a higher number but frankly, I would really not recommend it.
What's your reasoning for wanting such a long lasting KS in the first place?