Why does meiosis increase genetic variation in the offspring, but mitosis does not?

If you know how meiosis and mitosis work, this answer is actually quite simple. Here's a little recap on how meiosis and mitosis work before I dive into your question. 
Mitosis:
Basically what mitosis is doing is replicating your cells. Mitosis uses somatic cells, otherwise known as skin/body cells. These are just your everyday cells in your body basically, except for sex cells. It's actually what dust is made of. In mitosis, you begin with 1 cell, that has 46 chromosomes. This cell then duplicates, and then divides, leaving you with 2 cells with 46 chromosomes each. Now as a repeat, these cells that have duplicated are IDENTICAL.

Now onto meiosis.

Meiosis:
Meiosis uses gametic cells, instead of somatic like mitosis uses. Gametic cells are sex cells (sperm and egg). In meiosis, you start out with a cell that has 23 chromosomes from mom, and 23 chromosomes from dad. This cell then duplicates and divides like in mitosis, and then divides a second time. What you are left with is 4 haploid daughter cells. These cells are NOT identical, because a whole lot of other stuff is also going on during this process in which I did not explain since this is the short version. Basically, the sperm and egg combine, and then produce genetically different sets of chromosomes from each parent. 

I hope this helped clarify and answer your question. If you are still confused, I have included some of my biology notes on the topic which also includes some pictures if that helps you understand a little better. There are also a whole bunch of Youtube videos on meiosis and mitosis you can watch which can also help if you are still confused on these processes and why they are different and what is even going on during them.

Also, if you are wondering what my color coding means in my notes, pink/red is for vocab words/definitions and blue is just topics. I also used the blue and pink to differentiate between the chromosomes from your mom and from your dad.