It seems like this can easily be fixed by having people sign up for contracts that last more than 1 year or at least have some financial incentives that accrue

It seems like this can easily be fixed by having people sign up for contracts that last more than 1 year or at least have some financial incentives that accrue to employees who stay longer? For example, tech companies have stocks that only vest after 4 years. I don't see why in principle this couldn't be even longer. I don't know much about it personally, but I think the US government has a lot of benefits (like retirement) that scales with years of service. I actually suspect that at least for many big companies the problem isn't getting people to stay after training, it's getting the best employees to stay and getting lower-performing employees to leave. I think for this reason, most tech companies structure pay with a 4-year contract, which promises a high salary for 4-years (due to bonuses and stock), and then an implicit pay drop if there's no promotion after 4-years.