Also, credentialling. Even if you can't do the work and only stay a few months, Google or Goldman or another big name on your CV will still open many doors.
In Germany there is a culture of not changing jobs very often (at most every few years, but even this should not happen too often). So if you have a "well-known" name such as Google or Goldman Sachs on your CV, but worked there only for few months, it is a clear sign that you were fired and about every future employer will be very cautious.
Outside the Valley and startup culture most companies looking for software developers in the US view a tenure of less than two years as being short, and a series of them indicates "job hopping."
I'm not commenting on whether that's a good or bad thing, because good or bad it's simply the way it is.
1. What is most important in your CV is the last employer. If it seems that you were fired, this can cause problems with getting a (decent) job afterwards. If you were poached, it is because the other employer wants you badly. So in your example your new "last" employer becomes JP Morgan or Facebook. So this kind of problem does not occur here.
This does not contradict the fact that it still leaves/can leave a bad impression on impression if your were willing to be poached without a very good reason, since can be considered as a strong sign that your are not loyal to your employer and only hunting for money.
2. Poaching is rather uncommon in Germany. As I outlined in 1 being willing to be poached multiple times is a sign that you are not very loyal to your employer. Thus there is a strongly reduced incentive for employees to be poached.
Also for poaching to work, you have to offer lots of money - I'd be really surprised if there are lots of employers that are willing to start such an "arms race with money". Why not simply offer decent money in your regular job advertisements and get good people without any arms race?
Poaching might make sense if there you want to get "rockstars" that are not available on the "free job market", but can only be poached. But I'm also not aware of any industry sector in Germany where there exist employees that are treated as rockstars. :-(
Your vision is clearly limited to Germany, where poaching doesn't exist because there is no jobs and no strong companies competing.
Just imagine a random company in a tech hub. They host a meetup once... the days after the meetup, most of their employees have been contacted by recruiters and other companies who attended.
If they want anyone in particular and/or if any employee's salary is not on-par with their value, here they go! :D
money.