I think it is highly dependent upon the industry and job.

I've interviewed and hired dozens of other developers and never met one that knew the first thing about the industry we're in. We have a complex suite of products and to get someone up to speed will take investing a not insignificant amount of time. So when I see a large succession of sub-year stints I'm going to ask. I've hired many people who were contractors in the past, or who had issues with past employers, but I'm certainly going to want to know what the issues were so I can be sure they aren't going to have them at our company.

Both employer and employee want to minimize the risk of a quick exit as the hiring/job-seeking cycle is financially and emotionally expensive.

It depends who you work for and where you work. I work for a large aerospace company in the Midwest, and people still routinely work here for life. Many of those who don't stay in the same area, in the same industry, and work for contractors for the same company.

How many did you never hear back from after they looked at your job history?

I.e. how can you know this, unless you get jobs entirely through networking, in which case you're probably bypassing that sort of gatekeeping to begin with?

Never used networking, and the last few jobs I've had were applied to solely.

I am aware of a fairly large employer that looks at any stay < 3 years as suspect.

Most I talk to simply see it as the changing climate of employment, especially in the tech sector. We're all explorers and we like to move from thing to thing. It may just be a product of the industry, but I doubt I'm simply an outlier.

I didn't say I agreed. I just said that that mentality existed. I think long stays in established areas leads to organizational rot and lore accumulation as information and expertise is sunk into an individual (this could be mitigated by migrating around a company every couple years and forcing lore to be written down and documented).