Depends on the day/session, but I think it's more likely to be a logistics thing than anything. If I'm just headed out the door to do a solo workout around the neighbourhood I might just roll straight into the main set, especially if it's tempo or something not super hard to start with. I might do some strides or whatever if I have time and feel like it, but it's pretty much whatever I feel I need to get going on the day rather than any particular structure. Group sessions naturally involve a couple of minutes stop time while everybody gets it together, and for a lot of people those are the more important/focused sessions.
Also a master procrastinator, so if the workout sounds really hard you bet your sweet ass I'm going to take my time pretending I'm 'loosening up'