It's a standard that's been around for a long time, is in widespread use (how many digital music players don't support MP3?), and it's good enough as far as most people are concerned.
I don't have many albums in FLAC format, but the one that is most likely to have 'average length' tracks in it has file sizes that are probably quadruple the size of the average MP3 I've ripped on higher-than-average-mp3 quality.
I played around with bitrate/VBR settings with mp3 quite a few years ago, and with the settings I ended up picking, I couldn't tell the difference between that and the CD audio I ripped the mp3s from. If I thought about committing the time to re-rip all of my albums in another format, I would want to be able to hear a difference in quality first.
Furthermore, when mobile, most people are listening to music on their phones, which don't yet have (on average) "more space than they'll ever need", so I think the OP's question is about a decade "too soon" to ask, if at all. My PC's music folder has more in it than I can store on my 16GB Nexus 5, and it would be way more if it was all in FLAC format.
How it is easier to rip to FLAC than MP3? I know I don't use a popular method, but I basically open CDex, pick a command from a menu and the job starts with my usual settings. I'm sure I could reduce the number of clicks involved from two to one, but is this really a point you're going to argue?