Yeah but what does an Aranea have that a human sorcerer doesn't have?
Change Shape, which just lets them pass as human, +3 Cha, a variant of Web with uses-per-day and poison. Yes, this covers being a spider well, but
that's what Monstrous Spider is for. Aranea is about the Magic. A considerably better question is "what does an Aranea have that a Monstrous Spider doesn't?" And the answer to that is... Spider Sense, and casting. Oh, and Charisma instead of Strength.
Monstrous Spider has full BAB. Monstrous Spider becomes Large at level
2. Monstrous Spider, by 4th level, gets two improvements to Spider-things, both the webs and poison that Aranea get over Sorcerers. Or (Ex) Summon Swarm for spiders once per minute.
Oh, wait, Aranea can hold weapons. And coat said weapons with poison. Which is
awfully useful if you can make more than one weapon attack per round, and, indeed, only makes sense when you can actually hit things with weapons. Losing 2 BAB is not conductive to this goal.
If a Sorcerer wants to have Spider things and is willing to give up a level of casting, they get better mileage out of a level of Awakened Monstrous Spider than four levels of Aranea. If they want Spider things to be a Gish, then Aranea is extremely counterproductive to this because Monstrous Spider has twice the BAB. And gives Strength. And considerably more effective Spider-type things.
Aranea should be a Spider-gish in a can, not just "sorcerer, with a web and some poison in exchange for 1 level of casting." Particularly given that it
has a Gish ability to coat it's weapons in poison already. 3/4 or 1/2 casting and 3/4 BAB, with spider things being more coherently magic-linked, would be a world better than Aranea. Because at this point, all it gives for a spider-Gish over Monstrous Spider is +3 Cha, not even +2 save DC or a 2nd level slot, and the ability to use their poison on their weapon attacks.