not sure if this is the only reason
Since COVID and the rise of remote working, there appears to have been an increase in people holding multiple jobs simultaneously. In 2020, while working for a small financial services company, I was involved in a software release where a US-based colleague repeatedly disappeared midway through critical deployment activities. Eventually, management terminated his employment due to the frequency of these incidents and a strong suspicion that he was working multiple jobs at the same time.
Over the years, a number of recruiters and hiring managers have told me that this is not uncommon. Most companies are reluctant to get into lengthy disputes about whether someone is working multiple roles and instead choose to move them out of the organisation if they believe performance is being affected. I suspect that one of the motivations behind encouraging employees back into the office is that it makes holding multiple full-time roles much more difficult.
I also know of people who have successfully worked several roles simultaneously for extended periods. An old friend of mine has been doing three jobs at the same time for the last few years, a mix of contract and perm, the ones that expire , he gets another one. The demand for experienced professionals remains strong, and many employers prioritise proven skills and experience. As a result, graduates and entry-level candidates can find themselves at a disadvantage when competing for opportunities.