I am not grateful to my grocer for stocking milk in the dairy case. They set the price, and I pay it if I want the milk. If the price is higher than I am willing to pay, I do not buy it. Loyalty does not enter into the equation. If it did, the grocer might offer me lower prices for shopping only at that store, or additional services not available to first-time or fickle customers.
Paying a salary is zero risk for an "at will" employee. You are paying for the work, or at least for the exclusive on-demand availability. Whether that can be monetized to a degree sufficient to pay the salary is entirely attributable to management's effort and skill. If at any time you feel that you do not have the skill or the will to make the employee's labor cashflow-positive, you can fire them.
In contrast, the employee taking on a salaried position with a new employer is a much greater risk. That often becomes their sole source of income, they may be foregoing other available opportunities to take it, and may also be relocating to an area that is less advantageous for them personally.
It is for these reasons that I believe the burden is upon the employer to make demonstrable shows of loyalty to their employees first, to prove that they are trustworthy employers. Instead, they are often the ones to betray even the low default levels of employee loyalty. These days, I hear about mass layoffs far more often than I hear about employee strikes or walkouts.
I agree that we should mourn deeply the passing of employer-employee loyalty, but it has been fading for quite a long time. If you are a loyal employer, you will eventually see more loyalty in your employees. In time, they will even enhance your reputation via word-of-mouth recommendations. But reputation is slow to build and easy to squander, and US employers have utterly destroyed any reputation they may have had for loyalty.
I am surprised that you are amazed, but that may only be because you have never worked "at will" jobs in markets away from hot-spots like SV and NYC. Even just by reading HN, you should have noticed that the endemic career advice to maximize your earnings is to change jobs every 2-4 years, and only spend a longer period at a single job if a lucrative vesting schedule demands it. Employers do not retain and retrain temporarily unproductive employees, nor do they make noticeable efforts to retain productive ones with bonuses and pay rises.