The client needs of on-demand staffing is constantly changing. You don't seem to be familiar with the industry, so this is a quick breakdown of what happens:
People looking for immediate work (typically paid daily) go into a placement office and apply for work. The next day, there is a line of people that starts at about 5AM because when the labor office opens work will be given on a first come first serve basis. Generally, the expectation is that if a job becomes available a worker will take it regardless of what it is otherwise there's no point in being an on-demand worker (thus, a resume would be pretty silly). The jobs that clients request labor for are very low skill and often repetetive jobs that require little if any training outside of "here's what I need you to do, now go". People looking for this kind of work usually do not mess with online applications and it would be counter productive for the temp agency. The agency wants people who will use their feet to accomplish their goals, so the very first step in the screening process is actually getting folks to the office. Not online. On-Demand labor companies do not like wasting their time with applications from people who have shown no commitment to the concept and require a call back which takes valuable time away from the "on-demand" objective. Hence: no online application process.
If you are not familiar with this it probably seems very alien compared to traditional career positions, but it is a booming industry.
LTNC