The reason is that in the case of money invested, you deduct it off of revenue, and the net amount is what you actually made, and have to pay taxes on.
In other words, I buy a candy bar for $1.00, and I sell it to you for $1.50. I can "deduct" the $1.00 investment against the $1.50 revenue, and I pay taxes on 50 cents, because that's really the amount of money I made.
The money I pay for a movie ticket isn't being spend in an effort to make money.
The core principle in the tax code is really pretty simple. If you make money, you pay taxes. If you had to spend money in order to make the money, you get to subtract the money you had to spend, because you really didn't make all the money that came in as revenue. You had to spend some money to generate the revenue.
It gets complicated because people cheat, and the IRS is constantly trying to invent new regulations to disqualify the most recent very clever way of cheating, and congresscritters are constantly adding in new ways to game the system as a favor to their constituents, by adding deductions and rules that make no sense, but which benefit someone who might vote for them or contribute to their campaign.