But why should they? Doing that brings very little upside for MailChimp and a lot of potential downside if Google decides to blacklist MailChimp's entire IP range.
And then they have to devote engineering / moderation resource to routing emails to the correct IP shard.
Mailchimp has offered dedicated IPs for years - the code and operational processes around that are already established. Gmail also rarely blocks at the IP level, especially for an established provider like Mailchimp.
Ultimately, this is first and foremost a policy decision around the extent of fraud in ICOs generally, and secondarily related to many ICOs' lack of adherence to Mailchimp's anti-spam standards.
And then they have to devote engineering / moderation resource to routing emails to the correct IP shard.