The ads are very effective, but ad agencies are not filled with top tier people, you can train someone to run ad campaigns in a day. As a result, much of the button pushers at small agencies are recent grads with backgrounds not well suited to optimization.

A remarketing ad can be set up using exclusion lists to stop display after a purchase ( checkout page cookie), also most should be set up with limits. These are basic,obvious and easy things to implement. Attribution isn't always perfect but what you observed is more of a reflection of the high frequency of low tier ad managers. Many don't know any better, many don't care, many have no access to actually edit a client website (or know how)

It's the wild west still.

The example you used is also of a brand that I am overmarketed to, whoever is spending their money has some belief or knowledge of very broad targeting that I don't understand. I have not nor ever will buy blinds and my browsing behavior does not align with the obvious interest groups one would pick.

It's not just the small agencies that blow money, someone is patting themselves on the back about the 100,000,000+ impressions they are generating daily for selectblinds, but that funnel is going to be ugly when it's time to jazz up their presentation to show off that success!