Honestly this makes no sense to me, other than there aren't any monsters of those types at a lower level. But that's not true when the lowest level dragon in the bestiary is a fairy dragon which is level 2, well within the grasp of a 3rd level Summon Animal/Construct/Fey/Plant spell (if it wasn't a dragon).
I kind of want this for some lower-level cultists, to be able to summon really low-level aberrations or the like, but RAW I can't. I just want to know if there was a specific reason why this is the case. It just seems odd to me. I mean, a level -1 Giant also seems odd, but whatever.
(Since I'm the GM in my home games, I can change whatever. It is also house ruled that all the Summon * spells work like Summon Animal, but for the given creature type. But this isn't going to help my mildly-crazy occult sorcerer who likes to summon ABERRATIONS when she can only summon FEY in Society play. ugh...)
If I had to guess, I'd say this isn't much more than a flavor decision.
Summoning an animal simply sounds like a lower level thing to do, regardless of the CR of the summoned creature. Summoning an elemental might sound like a step up.
But summoning a dragon at lvl 1 is, well... even if just happened to be a CR -1 dragon somehow, that's still a pretty epic thing to do. Perhaps too epic for a lvl 1 character.
That's all just speculation. I don't think it will unbalance anything if you house-rule those spells as lvl 1 spells and follow normal rules for heightening as Summon Animal. That is... assuming any homebrewed stat blocks you come up with for lower CR creatures of that type aren't broken.