Don't we just need a single sentence that says "these people will be taxed 5%, and those 3%" and be done with it?
3% of what? I loan you $1000 and you pay me back $1100. Do I pay $33? Or $3? Or nothing? Do you pay $30 and then get a rebate later? If there's a rebate, is it $30? Or $33? Or do you pay no tax on the loan but get a $3 credit from the interest? Does it matter if the loan is for a house? Or for gambling? Does it matter if you give me shares of stock when I give you the money and I give them back when you give me money?
And that is ignoring things like gift tax and corporate tax. And deductions.
It takes 70,000 pages (I'm taking your word on that) to explain what is and is not covered under each tax and deduction. Given the way that some of the taxes work, particularly those related to business and investments, this can get complicated.
Much of this complexity comes about because of unfairness (real or simply perceived). First someone will point out that it is unfair to tax both this and that because they're essentially the same thing in that case. So they add an exception. Then someone finds a way to exploit that exception in an unintended way. So they add another rule saying not to do that. Then someone discovers that is unfair and we start the cycle again.
Finally, while people are the ones who pay taxes, the system doesn't tax people at different rates, at least not explicitly. It mostly taxes transfers of money in some form. For example, the first $9275 (after deductions) is taxed at 10% and the next $28,375 at 15%. So it's not different people who pay different rates, but the same person at different levels of income. This gives different effective rates for different people.
A system where different people pay different rates is unworkable because it forces people to pay ridiculous amounts if their income is close to the border. For example, someone who pays 10% on $9200 nets $8280 after $920 in taxes. If that same person would pay 15% on $9400, then the net would be $7990 after $1410 in taxes. Making $200 more gains you $290 less.
This is not to say that the tax code isn't bloated and over-complicated with redundant and obsolete provisions. It's just to say that we shouldn't get our expectations too high. An overly simply tax code would have its own challenges. At the far end, what would happen would be that instead of being statute driven (tax rules are written down in laws and regulations), it would become precedent driven. The effective tax rules would be hidden in court precedents.
The tax code can't be enforced as a single sentence. In practice, it's going to be longer. Decisions have to be made. Preferably they are recorded in such a way that different people are taxed in the same way.