I just really don't understand why anyone would want a 5.5" phone.
Why? If your sentiment is influenced mostly by the opinions of the crowd HERE, it's a very narrow sampling built upon herd-mentality that has long bashed anything that differs from Apple's currently-available offerings. You might as well not understand why anyone would want a Windows computer or why anyone would want to use Microsoft Office software. And yet, far FAR more people own and use Windows computers and Microsoft Office than the "superior" Apple alternatives this narrow sampling will always endorse.
Of course, this crowd HATED bigger screens while Apple still clung to the perfection of 4". Note however how readily this crowd jumped right on to 4.7" and 5.5" when Apple formally endorsed those new sizes in the products now available for sale. There's relatively little Apple bashing HERE about how wrong Apple was to roll out >4" screens, only bashing of the "other size" iPhone (the one that "I" didn't choose for myself).
The Android crowd has long been accustomed to having phones available with screens >6". To them 5.5" can look small and 4.7" can look too small. They are a BIG crowd of buyers who wanted a bigger-screen so badly they would live with Android over iOS. They have money and Apple wanted that money. Apple knew it's existing market would overwhelmingly buy whatever Apple decided to roll out in a new iPhone. So they get to re-sell the faithful and take a big bite into the major competition.
However the best answer to your statement is to simply sub in other Apple products. I really don't understand why anyone would want a:
27" iMac... or a 21" iMac
9.7" iPad... or a 7.9" iPad
15" laptop... or a 11" laptop... or any size in between.
There is no one perfect size for everyone. Lots of people are buying 5.5" iPhones. They are not wrong, nor are those buying 4.7" screens, nor are those clinging to 4" or 3.5" and hoping it will make a comeback.
I wear size 12" shoes. From my perspective, I might say "I just really don't understand why anyone would want shoe sizes other than size 12". Why do the shoe companies even make shoes other than size 12? Because the world is populated with people who are not me. And those people have money to buy shoes too.