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<p>In startup-laden areas like Silicon Valley, it is often the companies that get cycled through (i.e. most startups do not become successful and eventually close, or the remnants get bought by other companies after decline). It is not unusual for computer software people to change jobs frequently because of this. It is also not unusual for computer software people to change jobs frequently when a new startup doing stuff more interesting to them appears.</p>
<p>A key component to being successful on the job is the ability to learn something new when it comes along. However, that desirable quality is hard for employers to check for in typical interview situations, so that they often shortsightedly overlook applicants who do not already have experience in what they are looking for (but which good employees can learn quickly).</p>