Some have such terrible english grammar that ...

The bad language in the spam/phishing emails is actually intentional, according to the "experts."

Although it only costs a little to send emails, in order to "catch a fish" it often takes quite a lot of back and forth communication to get the hook set. Since they have to have hundreds - or thousands - of fish on the line to get a few that actually pay off, and some individual writing is required for a decent yield, there really is quite a lot of "work" involved.

By eliminating those bright enough to ignore anything with really bad grammar, they increase the odds that those they lead on will give them a higher percentage of "catches."

The spammers and scammers consider lists of those who don't respond nearly as valuable as the lists of those who might, and there are very active markets for transaction on both kinds - which means that sending a smart-assed reply back (to let them know how smart you are?) gets you on a list that they'll make a little money from.

A couple of quite analytical studies have given "return efficiency" estimates, based both on purely statistical analyses and also on traffic studies. I don't have a link handy but might be able to suggest a link to a couple of them if I get over being too lazy to look.

Targeted phishing (spear phishing) where they know specific persons they want to get to never have similar poor grammar (unless they know in advance that the boss they pretend has sent you the email is incredibly stupid?).

John