Probably because their employers think their worktime is too valuable to waste it with fieldwork. Often, they are overwhelmed with office duties, so that there is not much time left for fieldwork. This is certainly regrettable, but unfortunately reality.

I fully agree with Ronald. The work of researchers, regardless of discipline, is a purely intellectual work. Therefore, once planned and tested the sampling programm, field work can be accomplished very well by technicians, volunteers and students. Obviously it is imperative that researchers have cognizance of sampling issues in order to optimize the sampling, and properly analyze the data.

The fact of using samples not tested and not to show the sampling plan is a matter of professional ethics.

I agree with Ronald. As an academic, the expectations in terms of managing academic programmes have snowballed in recent years (in the UK at least). It is very difficult to find the time to go into the field or when time is available it might be at the wrong time of year. Since finishing my PhD most of the time I have spent in the field has been with students. This, in itself can be instructive since when you are trying to explain phenomena in the field it can lead to valuable insights. It is also possible to use students to collect data that can be used in publication (my colleague Jeff Ollerton has done this with pollinator data collected on Tenerife). However, depending on the field of study, it can be difficult to collect sufficient data and for that reason it can be useful to use data collected by volunteers or to mine historic datasets. Oleg, re the quality of such information, I think this is an issue but some organisations, e.g., the the British Trust for Ornithology, do take quality control very seriously. They do this by designing scientifically valid sampling protocols, putting on training courses, etc, see https://www.bto.org/research-data-services/data-services/data-quality

However, the BTO have been doing this for many years and perhaps other organisations elsewhere who use volunteers are not so clued up.

As others had said, the western scientists rely so much on data collected by the technicians and volunteers or even their masters and Ph.D students. They are often in the field briefly to explain some concepts to their students before field work begins and probably take one or two pilot readings. Since they are more concerned with office duties and analysis of data acquired, they spend little time in the field but spend more time in the office.