What makes you think lack of investment in IT & security is the main reason they get hacked?
Vice versa, NSA has virtually unlimited (let's just say unlimited means tens of billion dollars) budget invested in IT and security. They have the top resources there too. Do they immune from data breaches and being hacked? The answer is a big NO!
Hacking humans is quite often the easiest point of entry.
[1]: https://www.equifaxsecurity2017.com/
This is inexcusable in 2017. Hacking humans may be easier but it's up to Equifax to figure out how to mitigate that risk. "It's hard" doesn't excuse their behavior.
However this is not an excuse for Equifax to not put more focus and investment on their security.
Sure, nothing is totally secure against a dedicated, motivated attacker with unlimited resources. Thankfully those are few and far between.
Based on that Equifax has said that this doesn't seem to be the case.
People pay credit rating agencies to retrieve their credit reports, get credit scores, and open disputes, and the best way to do all that is online.
The very fact that all this data was accessed from their public site is very troubling. What's the chances this is a basic SQL injection issue? What's the chances they didn't invest in security at all?