Why don't Investment Banks work with Unicorns?

Why do investment banks raise capital? If Rivian Automotive can have Amazon lead a $700m capital raise, why on God’s earth would they hire a bank to lead the raise for them? It seems like the Unicorn Start-Ups wouldn’t want to approach an i-bank to lead a round for them if they have already secured finding from GV, Amazon, Accel, etc.. It has made question the legitimacy of the companies that deal with banks to go on a raise.

Can anyone relate to this question?

Is it safe to say that Unicorns don't need help from a bank to raise capital, so in turn, we (the banks) get clients from other non-unicorn start-ups (who are a big part of the market) to lead an M&A process for them. Maybe a founder wants an exit, sale to strategic, etc..

I get a lot of banks have other services like restructuring, DCM/ECM, etc I’m not talking about that. I think I answered my own question but I’d love to hear what others have to say on this topic.

https://techcrunch.com/2019/02/15/amazon-leads-70…