Because the trademark is for a specific category - there was already a company named Alphabet (with that trademark) owned by BMW and that didn't pose an issue to Google. You could start an alphabet plumbing or something. It's not like they own the word.
So, do they own the word Google? Can you use that name in other categories? Google plumbing?
Google being a made up word is probably copyright and trademarked... and probably could be more broadly enforced.
You cannot copyright a singular word, even a made up one, and sue anyone who says it the universe doesn't work like this.
Are you saying that Alphabet invest in only one category?
Having or not having a trademark for a category has nothing at all to do with where they can invest.