Well there's a few things to keep in mind about it:
1. It's not as if you spend 10k hours on something and suddenly a switch turns on in your brain and you're a master. Instead, it's simply the principle that as you spend more time with something, you become more familiar with it, and after a long time you will have a good understanding of it. How long will vary by person. 10k is just an easier number to remember for people to latch on to, sort of like how they say you should walk 1000 steps a day (which doesn't mean if you only took 950 steps today you're suddenly a fat slob).
2. It's not just about the time spent, but how you spend that time. For example, for engineering, just because you put in 8 hours a day at your job 5 times a week doesn't mean that you'll master the job's field in 5 years -- because so much of an engineering position is actually about attending meetings, write-ups (documentation), etc., and not actually about applying your engineering skills. The time spent doing "other stuff" doesn't count toward the 10k hours to become a master of engineering (I guess it does help you toward becoming a master of meetings, deflecting responsibility to other departments, how to request a bigger budget and get more time for a task, etc. though). For practicing, if you spend your time goofing off rather than focusing on your problem spots and thinking about how you can improve your phrasing, etc., then you're not really putting much time toward becoming a master. (Although it's not like there's no benefit. Spending more time with a piano means you become more aware of the instrument physically -- the feel of the keys, how the way you press keys affect the sound, etc., and of course messing around is fun and keeps you interested, so it's not really "wasted" time.)
3. I assume you've already started playing piano, so the time you're already spent can be subtracted from the 10k hours thing -- there's less time to go than you might think.
4. Ultimately, unless your goal is to become a professional pianist, a lot of "why" you play piano is for your own personal enjoyment, which can happen regardless of your level of mastery. (If you're a professional pianist, then you need the mastery for your livelihood.) You don't need to be a master of something to enjoy it, and there's plenty of music that don't require a high level of skill and yet sounds good and can be fulfilling (I would throw in most of pop music but people think classical pianists are elitist enough already

). Becoming the master of a craft is not the only "decent goal". It can be whatever it is that is fulfilling for you. For me, it's the challenge (and reward) of learning a new piece that's just slightly harder than what I think I'm capable of, so that I develop new skills and improve on my abilities, regardless of whether or not I'm good at playing piano compared with "the masters" or professional pianists (I'm not).
5. Similarly, it's not as if you need to be a master to impress people or for people to think you're good. At my university, because the piano practice rooms are so often full, I'll usually end up playing on the public piano in a large lounge at the student center. (Out of deference to the people around, I try not to repeat the same section over and over as if I were practicing, but play the pieces once through and try to think of how it can be more musically expressed, etc. rather than working on the more technical aspects.) Even though the pieces that I play are by no means considered master level -- pieces like Grieg's Wedding Day at Troldhaugen, Sibelius's Romance and Chopin's Fantaisie Impromptu -- and I by no means play them like a master, they'll often garner applause from whoever happened to be there. (As another aside, I feel like people are more appreciative of my FI now that I've slowed it down a lot and tried to work on better phrasing and clarity, rather than my previous way of running through it sloppily at breakneck speed.)
6. And finally, as for how to practice, you should
not try to keep repeating a piece until you don't have a mistake before moving on. The human brain has limited attention span. After about 20-30 minutes or so (depending on the person), you're not really concentrating on playing it right anymore, and you're basically just waiting to make a mistake and getting frustrated over it. Just note where you messed up, move on and save it for the next practice. When practicing, you should focus specifically on the sections where you have trouble, and go over those, rather than playing each piece from beginning to end. Do it in small bite-size chunks, perhaps even just one or two measures at a time, depending on the person. Bernhard on the forums had some great advice for practicing, which you can find here:
https://www.pianostreet.com/smf/index.php?topic=12590.0