Spammers are better at SPF, DKIM, and DMARC than everyone else

I think the problem isn't necessarily that adding DNS entries is hard (especially compared to the rest of the process of hosting your own email), but that getting a clear overview of what email tools an organization uses is difficult.

You need IT to list all of the reporting tools, customer service to tell you about their support system, marketing to tell you about their mailing list tool of the week, the sales guys to warn you they're using this new AI email enhancer, and somehow get that shady email forwarding service the CEO uses to give up their mail server IP addresses. Then you need to figure out how to get coverage for all of those tools and keep on top of them whenever something changes.

A lot of companies promise to do great things for you if you just enter the email address you'd like to send email from, and a lot of people gloss over the important details because those sound hard and when they tested the tool on their personal email it worked fine so that's probably unnecessary anyway! Managing email for a corporate domain can be like herding cats.