I've never owned an 808, but I've owned at least 18 hardware synths and samplers, as well as Live, Reason, Rebirth, and lots of softsynths on iOS.
I hate softsynths on OS X because it's not stable enough. I have two synths that were made in the 1980s and they don't need any software updates to run. You just plug them in and turn them on.
Hardware lasts for a very long time. Software, you don't really buy it, you rent it for the brief lifespan of your computer.
Emulating analog gear is difficult for the same reason that writing software which simulates any real-world process is difficult. TLDR: everything louthy said.
Even emulating digital gear, which logically should be much easier, is still a challenge. I have a Korg Wavestation and a Korg M1. Both are 100% digital. Both are well-emulated in Korg's popular and well-regarded Legacy Collection. But well-emulated isn't the same as exactly reproduced. And Korg supports the Legacy Collection now, but they won't forever.
I can't even trust operating systems to work properly when I install them. I'm not risking my ability to make music on the incredibly widespread incompetence of programmers, even at the best companies on earth, when all I have to do is put down less than $200.
Look at the bug trackers for any open source project from Apple or Google. Apple's still got WebKit bugs around arbitrary cookie resets from at least ten years ago. Or look at iTunes. The demand for programmers is so intense that you can be an utter incompetent and still have a terrific career.
If the best companies on earth can't get their shit together, how on earth is some music company going to do it?
Also, no two 808s ever sound exactly alike. The 808 is still unbeaten when it comes to claps and kicks.
Emulations are getting better and better, but you're never going to get a picture of a sunset which is more beautiful than being there. The only truly, unequivocally successful emulation project ever is the x0xb0x, which "emulated" the Roland TB-303 by taking it apart.
We have two--count 'em, two--analog electrical engineers with too much time on their hands working on this project. An original TB-303 was diassembled and reverse-engineered to verify the schematic. The original transistors were analysed using big expensive curve-tracers to determine their characteristics. We measured every waveform and counted every cycle to give you the best sound...its just like the original because it IS the original.
http://www.ladyada.net/make/x0xb0x/