Bottom line is we have to look at it from two directions.
First is from your side of the coin. You want to be a climber. That is laudable and worthy.
However, you are not a climber right now. In this business the way you become a climber is on your own time.
Companies will give competent ground workers opportunities to grow and learn and to become climbers but they cannot afford to put you into a climbing position and then teach you.
Think about it in these terms, would you go to a surgeon who has assistants who have been to school but never seen a human patient before? How about a Veterinarian Assistant who "Challenged" the credential process and is now paper qualified to be an assistant?
It seems that is what you are asking for, expecting to be hired as a surgeon's assistant without ever having done the job.
Suggestion, look for a gopher or other entry level ground position first. You can launch into climbing from there. If you are as driven and as competent as you say you are you will move up quickly into the roles you desire.
If you approach me looking for a climbing job then I probably won't have much of a conversation with you. If you tell me you really want in on this industry, want to work hard and learn as much as you can and ask how you can get started on that path then maybe we could have a conversation and find a way to get you on board.
I submit this, a climber who is not first a groundsman is not a complete climber. And a groundsman who does not know the climber's job inside will never be an effective groundsman.
'nuff said there.
Now, the other side of the coin has pretty well been explained, If I am looking for a climber, I NEED a climber, I am not looking for an almost climber, I am not looking for an unhatched egg of a climber, I'm not looking for a "when I grow up I wanna be" a climber. I need someone who IS a climber.
The reason I need a climber is that work is not getting done. If I have to train a climber from the ground up then not only is the work not getting done, it is costing me more to have it not get done.
Ask yourself this, if you knocked on a customer's door and told them you could remove their tree for them for $20/hour but you don't know how long it will take because you really don't know what you are doing yet do you think that you will get many customers to hire you without reservation? More to the point, do you think you could pay your bills?
An employer IS your customer and they have every right to expect a fair return on the money they spend to have you deliver your services.
Customers pay you for what you CAN do and offer you opportunities to do more as you grow and prove yourself competent.
Employers OWE YOU nothing, and as long as you feel you owe employers nothing then, well, you'll find yourself on the outside looking in all the time.
So, what do I do about hiring climbers? I don't. I hire ground people and make it very clear, "you will have opportunities to climb, but you must take the initiative and show me that you are interested, willing and able. I will not drag you or push you through the process. If you want to learn you will seek opportunities and make it happen. I will help, but you need to make it happen."
So far, none of my recent "climber wannabes" has progressed beyond ground worker status.
Not that they aren't capable, just not as interested as they thought they were.
You want to impress the hell out of an employer, be willing to muck out the stalls, come to work with the attitude and corresponding actions that say you'll do whatever it takes to become the best you can, show up with your own gear, preferable showing signs of good use, start climbing trees on your own for fun, learn how to move in the canopy, find other tree climbers and spend time with them.