This is a big reason for sure. As you can see in this post, those IP addresses can be hit or miss depending on the provider. At Postmark, we don't really believe in dedicated IPs for all customers. We think that our customer base should not include any bad actors, and instead manually approve every customer to ensure our entire CIDR ranges are clean. The benefit is not just clean IPs, but clean IPs that have an incredible transactional-only reputation with the ISPs. This is how we are able to delivery so fast to the inbox. We only really believe in dedicated IPs for higher volume senders, especially since reputation is moving toward the domain more and more. I wrote about this six years ago, and it is even more true today (https://postmarkapp.com/blog/the-false-promises-of-dedicated...).

At the same time, if you are willing to install and manage Postal on your own servers, it's not that hard to maintain your own IP with a great reputation. You just need a good hosting provider (probably not AWS), you need to set up your infrastructure like DKIM, SPF, DMARC, rDNS, and Return Paths, and most importantly you need to maintain good engagement (low bounces, high opens). At a glance, Postal looks like a nice option if you want to do it on your own for cheap. You just might lack the stability, support, maintenance, and performance that goes behind an ESP.