How so? As a feelancer, I enjoy coming for a specific task rather than getting allocated to a "pool" of developers, though I also get offers from big customers to join into another project after completing one. If I only consider long-term projects from banks etc. my skill set would be much smaller. When working for start-ups, you often have dedicated budgets for a particular customer project of theirs or other time limitations, but the scope of your work can be much broader.

Edit: also want to point out that if you only have worked in greenfield projects where you could grow with the particular tools and practices of that project, you don't know anything about writing maintainable code

It's not really a clear sign of anything. Many other things could cause that, such as a desire to learn new things often or changing jobs to increase pay after gaining experience. Technical work can be interesting and exciting at first, but become tiresome after a year of repeating the same shit.

Or someone who lives in SF and likes doing startups, or someone who is terrible at finding a position that is the right fit for them, or someone who is terrible at recognizing terrible working environments, or someone who...