It's a matter of cost of material offsets:
1: Doing it again because you used cheap inferior material.
2: Lower skill level required to get excellent results.
3: Lower additional material requirements.
While simple cheap concrete block sounds good it will fail over time if not done the way required, IE foundation, rebar, concrete fill, pinned back into the slope, and proper drainage.
It also takes a good skill set to be able to set block properly, and it's not fast unless your very skilled.
Railroad ties or any landscape timber will also fail over time, they always do, and even quicker if not properly installed, IE rebar pinned to the ground, back into the slope, and to each other.
Most people do not have the proper tools (BIG DRILL) and long auger, and if you've never drilled a RR tie, they don't drill easy.
Most RR ties you can get now are junk, they are pulled off rail lines and are on there way out that's why the RR changed them out, and new RR ties are super expensive!
This solid block states 36" ( higher walls require hollow blocks and rebar and concrete fill) and at $605.58 a pallet it's not the work price out there.
there are smaller lighter versions of these too.
https://www.homedepot.com/p/Pavesto...k-45-Pieces-30-2-sq-ft-Pallet-83675/202365278