Those who stay in the Novus Ordo, FSSP, diocesan Indult, etc. at least don't have a problem with the Conciliar Church. They see no major Crisis, just a few "implementation hiccups" that took place at Vatican II. So their truck/commerce with the Conciliar Church is understandable.
Some (certainly not all) who attend the FSSP do have a problem with the conciliar church, and the new mass, and they do see a major crisis. The issue of why they do not attend an SSPX or Resistance chapel may vary, but it could be that despite the crisis, they still think that they HAVE to be attached to the "official church." I'm just relating my experience of having attended an FSSP parish, though I wasn't very involved there.
Or, they may have had a problem with the SSPX in the past (as I did), or maybe no other option is available where they live, such as no SSPX, independent, or Resistance chapel is available (or something like that).