Equipment is more than you think. Trucks with cherry pickers or spider lifts are a couple hundred grand each, plus they need to be maintained and replaced. And you probably need more than one. So now you're deep into 6-figure debt, and that means big monthly payments. That's before you even get to the costs to store the equipment safely, leasing an office, skilled workers, etc. And yes insurance and bonding, which, you can cause some major damage to multiple structures at once, so that's not cheap at all. How about marketing? Do some billboards or commercials? Tens of thousands more.

I've worked in commercial construction (which absolutely is a cash generating machine) and tree trimming services are like a mini version of that - you need a lot of upfront cash to make it happen and get a stable cash flow going before it's solid and profitable.