This is an established enough business strategy there's a name for it: "fast follower." (See http://www.businessinsider.com/youre-better-off-being-a-fast...).
After that, yeah, a lot of following, then again for the GUI introduction era everyone was following Douglas Engelbart, and Xerox PARC for the graphical part.
Microsoft's failure was in hesitating to change with the market, not a failure to have products on the market.
Tablets, you can make a case for, as above, I guess, but again, the leap with the iPad was so great that it wasn't just an iteration but a leap, leaving it open for a fast follower to come on (as Google did there too).
Apple and Google didn't have Windows Mobile 6, 7, and 8 being planned and developed when iOS or Android were released. Microsoft did. It's slower to turn a ship around than it is to start one going in the right direction in the first place.
And besides faster processors and a slicker UI, what exactly puts Android in a different market than Windows Mobile? Serious question. Both allow you to develop, download, and install apps. Both are pocket computers with a cell phone built in, built on the same paradigm that Microsoft ushered in throughout the 90's (give users almost complete control, let OEMs do whatever they want on the hardware side). Windows Mobile has touchscreen support. Both can/could be gotten for very cheap or very expensive. Android seems to be, for all intents and purposes, a straight-line evolution of Windows Mobile with a Linux kernel (yet still closed-source where it really counts).
What supports my claim that the market was created by Apple and Google? You cannot exclude "faster processors and a slicker UI", as you put it - nor what they're a part of. It was a revolutionary change in user experience.
Your point about neither Google or Apple having involvement in the industry is a good one and it's not completely uncommon for stories of disruptive innovation - which this clearly is...
Which is not to say they can't execute a fast-follower play successfully anymore -- see XBox and Azure. Just that they don't nail it as thoroughly and consistently as they used to.