Before I answer you question, here's mine: are you asking "how people have seen it so many times" or "why people have seen it so many times"? The former is asking about the means of going to the musical and the latter is asking about the reasons of going to the musical. Depending on which question you're asking, you're going to come up with a different answer.
If the question is "how," then this is the answer: people live in NY or close to it. They can travel and get to the theatre easily. They normally will try lotto and if they win, that's $27. If they lose and still want to see the show, they can get partial views for $37. If they're a student (or someone who still has a student ID), they can get rush tickets for $27. If they accumulated enough Idiot Nation points, they can reserved lottery seat tickets and pay $27. If they are able to find discount codes for tickets or to buy tickts at the TKTS booth, they can pay between $27 to $85, depending on the offer. There are tons of ways to buy tickets to the show without having to pay full price. Hell, the show itself gave free tickets last week.
If you enjoy the show that much, then you know the different ways to be able to see it without losing an arm and a leg. I have seen the show about 25 times. I've only paid full price for a ticket twice - once for the first week of previews and once for opening night. All the other times have been discount codes, partial views, or lottery/rush. I know of people who have seen the show 30, 40, 50+ times. They aren't rich; they're just really good at lottery and/or getting partial views.
Now, if the question is "why," then my answer only speaks for me. I go see the show numerous times because it's never the same show. During previews, when the full original Broadway cast was still there, they were still tweaking the songs and the blocking. One show would have this one version and two shows later would be another version. After opening, so many understudies and swings have gone on that they add another something to the show.
Look at this past week. Understudy craziness. How many people that I know rushed to the theatre after hearing Christina was on for Whatsername or Declan was on for Will? A lot. Which goes with another reason of "why." The cast. The cast is amazing and they put their heart and body into the show every time. Even when they're not feeling too well. Plus, they're really sweet and friendly... and a bit zany, actually. Lots of quirks. No one is really straight-laced, even ones like Stark or Tony. I like supporting all of them and cheering them on, even after they leave the show.
Also, because so much it going on at once, you have to see it more than once to notice everything. Heck, even after 25 times, they still do things that make me go "Oh. I didn't notice that before." For example, the first time around during "Favorite Son," you might pay full attention to Wallace and the girls. The second time around, you might pay some attention to Stark and his reaction. The third time around, you might notice the madness that is the couch with Esper, Dec, and Miguel.
So, there ya go. You don't have to be rich to see the show more than once and everyone has their own reasons on why they would.