Because it takes extra programming, and achieves nothing important.
What, you think someone who's been abyssea leveled from 30-90 will be worse at their job than someone who leveled it "properly"? Sorry, no. Crap players will be crap players, good players will be good players, doesn't matter if it took them 2 months to reach Lv90 or 2 days. You don't need to slowly grind a job to "learn" it. Hell, A few weeks ago I leeched my Corsair to Lv90. I have solid melee and ranged TP sets, three different WS sets, quick draw set, idle/kiting sets, PDT and MDT sets, phantom roll and random deal sets, and so on and so forth. My Skills are capped (and merited), and my Cor specific abilities are fully merited. I'm a significantly better corsair than those who fulltime the same set of shit gear, even if they slowly leveled their job so they could "learn" how to play it. I will be more useful and output more damage when required than those players. I know when and how to Quick Draw, how to maximise it's damage. How to get the most out of my rolls, which rolls are the best for which situation, which SJ is the best for which situation. I basically know the job inside and out. Did I need to level slowly to learn that? Hell no.
And yesterday I leeched my warrior to 75. Later this week I will finish it up to Lv90. Again, I will be a much better Warrior than any of the shitty full-pearle Warriors, even if they "leveled slowly to learn the job". That's just that. I'll say it again: There are good players, there are bad players. It doesn't matter if they lvl a job in 2 months or 2 days, at Level 90 they will still be good players or bad players.
"be more enjoyable by exploring new area's"? You may find that enjoyable but not everyone else does. You're basically assuming that everyone will find the same things in FFXI enjoyable. That's a very bad assumption to make. This point is thus fairly irrelevant.
And guess what? A new player doesn't have to leech his job to 90 in abyssea. They are more than welcome to explore other zones and do FoV. You are also free to level how you want.
tl;dr:
You don't like it? Great, don't do it. Not everyone enjoys the same things.
The amount of time it takes to level a job does not affect how well a player will be at that job 95% of the time.
Protip: You learn a lot more about your job by reading sites like BlueGartr, Wiki, Allakhazam, etc and asking your linkshell or friends than you do by "leveling properly".