The alternative explanation, that they roitinely use [your name].[your last name]@gmail.com as their email, and don't realize that it never works is... unlikely IMO.
I can easily imagine there are a bunch of older people who do not have email addresses, yet just about everything requires an email address, even if it's not necessary for whatever service, so people have to either make up an address, or incorrectly remember what the address was that their children or grandchildren set up for them.
Either way, that doesn't excuse the companies that spam innocent people.