My sister works in the advertising industry, where it seems few people spend more than 2-3 years [edit] in a firm before moving on. The big advertising firms tend to be in public ownership and and have apparently systemised the creation of advertising and marketing content, such that it doesn’t matter who is in the job or how long they are there for. I was pretty surprised by that - managers are well paid and grind creativity out of younger employees. Maybe I shouldn’t be surprised - maybe those are just well run businesses, but I’m helluva glad I don’t work for one!
The average tenure of someone, particularly a junior person, at an ad or media agency is around 1.5 years.
The reality is that there's always opportunities at other agencies where they'll pay you a bit more than you're currently making (which is still less than paying someone there already more) and give you an inflated title.
Due in part to the nature of the industry, tasks are made to not be dependent on a given individual, and people float around because agency teams are never at equilibrium. They either have too few clients for the team and need to let people go, or too many/too big clients and need to staff up.
Source: worked agency side for most of my career and managed a bunch of people. I'm brand side now at a company with a significant percentage of employees at or over the 10yr mark. There's a reason I left the agency world.
