You want a tax break for being single and not having kids? You already should have less expenses, therefore you are already paying less tax than a person with kids.
For example, let's say hypothetically you and Mike have the same salary, but Mike has three dependants. Let's say every person needs $50 a week to eat and there's a $5 tax. A person single with no kids pays $5 in tax. A person with three dependants pays $150 on food $15 tax. Not only do his expenses go up, but the goods and sales taxes he pays over the course of the year will be higher than the goods and sales taxes you pay.
At the end of the day, you will have far more financial freedom with your wage, and a higher quality of life.
Yes Mike choose to have kids, but his kids don't bring in an income, so he is paying more tax despite having the same income. The tax break isn't to make his life better than yours, it's to try bring up the quality of life for not just Mike but also his kids, to prevent them falling into poverty.
This is of course simplified and the numbers arent accurate but it is to demonstrate why there are tax breaks for having kids.