Why do people treat retail workers like they aren’t human????

I’ve worked in a grocery store coming up on two years now and yesterday was the worst experience I’ve ever had. I was watching self checkout and my store is overdue for new self checkouts so our machines are 20 items or less. This woman comes with a $300 order and I suggest it would be quicker to use a register and tell her there isn’t a line on the register nearest us and she flips out on me saying why don’t I want her to use self checkout and I’m like,,,,, just use it at this point I don’t wanna argue with her so I’m about to just let her use the self check anyway and she stomps off grumbling about how I make this so difficult. So I go to help my coworker bag because her order is huge she had two carts and she is just screaming at me and telling me she doesn’t want my help I’ve helped enough already. Then there’s a problem with her payment so I have to come back over and she’s screaming and calling other customers douchebags calling my coworker a dumbass and is so pissed I’m over there helping so she can pay and leave! And she finally leaves only to come back in and scream at my manager about me and the cashier! We didn’t do anything! And she’s yelling about you don’t know what people are going through which I found funny because I have trauma from verbal abuse and she’s screaming in my face you don’t know what people have gone through! And that wasn’t even an hour into my eight hour shift and it took so much out of me I just wanted to cry for the rest of my shift and my heart was racing when I got home and I couldn’t sleep last night I was shaky I just wish I could quit this job but I don’t have options right now :( and I have to go back in an hour