I have two arguments against manifesting.

First, I believe that Earth is most likely a spiritual training program and we come here to experience challenges. In heaven, manifesting stuff is easy as pissing and consequently the experiences you have there do not challenge you. The intentions behind manifesting are often nebulous, but I suspect most people do it in hopes of making their lives easier. By doing so, they are in many ways missing the entire point of why they've come here. You are likely trying to circumvent challenges that you yourself set in place.

Second, why manifest stuff in the external world? If you are going to manifest, I think you should begin by focusing on the internal world where you already have a great deal of power. If you want to be happy, why not manifest happiness directly into your being? If your intention is to feel special or powerful, then you're wasting your time because you already are special and powerful. If you're chasing feelings, why not manifest the feelings you want directly?

We know that manifesting is possible; in a sense, I manifested this reddit post into existence via an act of concerted will. But when you manifest, think carefully about what you are doing and why. I think that most people trying to manifest probably need to go outside and touch grass. Or maybe fall in love, get a better job, meet some new friends, and so forth--physical stuff that everyone can do. Because that's really why we're here.