"When it comes to build versus buy, the frequently repeated but rarely followed wisdom is good advice: if you're a technology company, vendors usually generate significant value if they're outside your company's core competency; within your core competency, they generally slow you down."
Every startup I've ever worked with has had engineers who advocated "build" for tooling that fell both outside the eng team's workflow and our company's core competencies.
I think part of it is that things like marketing automation or customer support software—things that are vital for most teams past a certain scale—don't seem technically "hard" enough to justify the price-tags, particularly to engineers who've been isolated from other parts of the business.
Without fail, every time I've been part of a team building an internal tool that we/another engineering team won't use, it goes poorly. I've spent way too long learning about the ins and outs of email deliverability so that I could fix a bug in a hacky email platform, when we could have just bought a platform that works.