Basically because the size of the FIB depends on many factors. On modular platforms it's scale is per MPC (linecard) and the RIB depends on which routing engine is installed. On top the scale depends a lot if it's just pure IP routing or you have MPLS with L3VPN's, EVPN or similar enabled - just giving an scale number in a datasheet could risk having customers buying a device which ends up not being able to handle the expected scale.
Therefore, ask differently: I need a device which can scale X FIB/RIB entries including 100 VRF's with Y number of 10G ports and Z number of 100G ports. What would be the initial best device to look at?
I saw that you asked in another thread what would be the cheapest device doing full routing table - but the VRFs and interface possibilities are missing to provide a proper suggestion. You could also go to your preferred Juniper partner to help select the right device for the task.
Let us know what you are in need of and we will try to help you narrow your search 🙂