You can get Quina before going through Gizamaluke's Grotto, and having a 4th would make every boss battle easier up until the current runs get him/her. Why not get him/her early?
A number of reasons.
In RTA runs, having Quina on Disc 1 doesn't actually help anything, and actually hinders you on Beatrix 1 because having Quina can make jump skip harder or impossible to do depending on who Beatrix hits. Additionally, Quina offers almost no value on the Gizamaluke fight, as all he/she can do is heal, which Vivi is already doing anyways.
Getting Quina on Disc 1 requires going into the marsh on Disc 1, which means it's an extra trip that we don't necessarily have to make. Additionally, you have to run all the way in, and all the way out of the marsh if you get Quina on Disc 1. I believe with all the dialogue accounted for, it loses ~2 minutes.
And finally, getting Quina early would sap a LOT of forced EXP from Zidane/Freya. The forced mage fights in Grotto and Burmecia, the encounter in Cleyra trunk (if you get one), and then ALL of the forced soldier fights during the Cleyra invasion. So you'd end up with Zidane/Freya (and Vivi) being a noticeable amount weaker overall, and all you'd gain from it is a couple more levels on Quina. These couple levels on Quina, and the small bit of survivability he/she gains from it, doesn't justify the ~2 minute timeloss and the EXP stealing.
Quina on Disc 1 is really good for Segmented, though, because in Segmented you typically get Limit Glove and use that to oneshot Giza/Beatrix 1/Antlion/Beatrix 2. We don't do this in RTA runs because it is very, very inconsistent.
