Pitch for why A2A?

You're completely missing the point of the question - they're not asking you why you want to be an associate, but rather why you deserve to be one given you have no experience in that role. Examples of good points include, but are not limited to (assuming they hold true in your situation):

1. Despite only being an analyst by title, you've been in investment banking for 3 years now, and have built sufficient goodwill / credibility in your group that you're effectively running as an associate in your current position
2. As the [most experienced] analyst on the team, you've been tasked with spearheading the initiative of mentoring new analysts and teaching them the ropes
3. You've fully grasped the financial modelling / analytical aspect of the job, and have been working hand-in-hand with mid-level to senior bankers on driving execution, idea generation, and overall strategic critical thinking
4. You're the "go-to" person in your group when other product / coverage groups have questions / requests; demonstrates that you've proven you're reliable, plugged in, and built a brand around your reputation
5. You're able to maintain strong interpersonal relationships internally; you know how to manage group morale, push back on certain requests (or able to provide suggestions on how to make workstreams more efficient while achieving the same end result), and your voice is respected in your group
6. You're given speaking roles in client meetings, and seniors can comfortably rely on you to address questions that are a bit too "in the weeds"

There's a ton more that I can list, but the whole point of the question is meant to address what makes a good associate, and why you think you fit the bill.