not that you shouldn't put time. But that you shouldn't put time you can't afford to lose. a chick will focus solely on you but only if you make it her best option. to do that, showing high value is only half of the equation. the other half is to also show her that if she doesn't bring in anything of value to you to the table, she won't get your presence. And you let her choose. If you don't bring that second part, why would she focus solely on you when she gets the benefit of your high value presence (first part of the equation) without the requirement (second part). That would be the better option in her mind. She'll save her devotion for the guy who won't put up with her BS and keep what you're willing to give her (the guy you want to be in that situation).
it's not that they can't focus on you. it's that they don't have to until you put them in that position. Imagine if you met a chick, dope as fukk, classy, smart, perfect body, angelic face, virgin, willing to go all the way to please you, support you, etc. but on top of that, willing to be sexually exclusive to you even if you want to mess with other chicks. what would stop you from going to other chicks? it's the same thing but in reverse.