Because it's nowhere near the same thing. We aren't whole-heartedly praying to the primal in question, and neither are we feeding it tons of aethercrystals to pull it from the other 'dimension' into ours. Though that doesn't mean there weren't other explanatory lore problems with the system... They could potentially, and logically, actually work.
As I understood the lore, having gone through the SMN quests fairly recently: First we are working with
*primal energy* which Y'mhitra called the "ASPECT of the avatar," not the actual primals themselves. We used rituals which shift our own aetheric balance toward element of the primal in order to summon this shade, which becomes known as the primal-egi. (Egi meaning "the essence of.") We do not summon the actual primal (either to defeat it or after we have mastered it), we go to places where they have been or where there is similarly aspected energies to draw upon, and call out a shade with their likeness, made from our own internal aetheric reserves.
Second, we're bending this (initially hostile) 'energy' to our will by defeating it. We aren't asking it to protect us, we're commanding it, and if I recall and read the quest-text correctly, it has no choice but to obey as it has "submitted to your will." This isn't a "friendly fight to get the summoned monster to respect you" (as seen with Rydia in FF2/4) this is a show of dominance and then an overwriting of will. Y'mhitra says that you are the "master" of Ifrit-egi, and then you must claim Titan as "a servant." She also states "A summoner employs the savegry of the primals to conquer the primals themselves."
I'm curious as to whether this was why Tristian's enkindle didn't work - since he was not technically its master. Tristan was simply "given" the essence of the elder primal Belias-egi by the Ascians. Anyway, wrong forum for that question.
Third, some primals are simply too powerful overall (even when we can defeat the actual primal) to do this with. Because Ramuh-egi requires more aetheric reserves than we actually have, the summoning fails. (Which is why we get Trance, as a method of infusing ourselves with the essence of the primal, rather than trying to summon it externally.
All of that said, we have since: A) Absorbed power from Midgardsormr, B) Absorbed power from Hraesvelgr, and C) Grown more powerful overall. It's entirely logical for them to conclude that we are now strong enough to summon Ramuh-egi (though since it's just an appearance thing, they could give him similar/the same skill functions as Garuda), etc. They could also find that oops, we're now strong enough to summon up to Bismark, but there's an "unexpected" limitation where you can only have 3 active egis at a time. (Using a different egi than what you have could possibly require going back through the initial duty, for example, or could be as simple as selecting summoning spells "prepared," as mages do in D&D, in an interface with some item like the transmog system we already have.)