I'm pretty sure there's some limitation regarding that. It's not only Toei and their Rider movies.
Look at recent Gundam games with SEED Destiny. One of the VAs decided that she wouldn't reprise her role after some problems with the series' script writer, so the character Cagalli is often just getting cut from new games or staying mute all the time, rather than anyone getting clips from the tv series for her.
Even some games series that actually have old voice clips for her character just are keeping her mute all the time now in voiced sections. They'd probably have to make some contracts with the actors if they intended to use their voices in new productions not covered by their original contracts, even if there was no new recording. If they need new contracts, they're pretty much better off actually getting them to record something or not using them at all.
Newer series might be different though, if they're able to use the stock clips record for the Ganbaride game, like Decade apparently did to Ixa and Psyga (who didn't keep the original movie voice anyway).