Comment by brightball

1. There are a lot of domains out there and all of the people who own them aren't necessarily technical enough to setup DKIM on their mail server. Ideally those people are using some type of service. SPF is much simpler in this regard.

2. This is a rather famous story about it happening.

https://www.wired.com/2012/10/dkim-vulnerability-widespread/

I have no idea how widespread the issue is today but I had to do some analysis on it when I worked for dmarcian ahead of the Anti-Phishing Working Group conference and we found that a significant percentage of email from known malicious IPs associated with reported phishing was passing DKIM. Key rotation removes the problem. Many services like ProtonMail and Sendgrid will set you up with 2 CNAME's for your DKIM keys so that they can rotate them for you automatically.

3. Domains send emails from multiple servers. Sometimes dedicated email servers, Google/Outlook, Sendgrid, email marketing tools, etc. A receiving system has no way to validate whether any of the tools sending email claiming to be from your domain are actually from your domain. The first time you look at a DMARC report for a domain that's been around for a while, you will typically see that 90% or more of the messages claiming to be from your domain weren't from you at all.