As explained by a friend of mine

Boomers: (two types)
1 – those who work because they refuse to retire because they “love to work” and are financially secure;
2 – those who work because of financial insecurity;

Either way those management and upper level positions are not vacated for the next generation (GenX).

GenX: stuck in middle management and senior staff positions due to Boomers not retiring. When a new position becomes available, whether previously held by a Boomer or otherwise, GenX is often not selected to fill the position due to now being too close to retirement age which means companies/agencies pull new management and upper level positions from Millennial candidates to “get more bang for the buck”. GenX is expected/required to then train their new managers.

Millennials: often selected to fill vacated Boomer jobs and middle management/senior staff level positions thus vacating many entry or low-level jobs.

Gen Z: in lieu of working in the service or manufacturing industry with its minimal pay, irregular schedules, no benefits, and abusive customers, Gen Z is able to pursue the entry and low-level positions vacated by Millennials which provide more stable hours, benefits, and generally better work conditions. This leaves a large employment gap in the service and manufacturing industry as

Gen Alpha: not quite old enough to work; thus some states have reduced the employment age or relaxed child labor laws.