If your instrument is manufactured once, drenched in potting compound and never serviced, absolutely, solid wire is ideal.
Other than that, right, fine, it doesn't need to be as extreme as potting, but vibration from transportation and ordinary handling will shake the cabling and fatigue solid wire. You need quite a lot of mounting to prevent that. And if nothing else, assembly and service will flex it plenty.
It's also redundancy from stress raisers. Stripping solid wire correctly can be difficult. Nicked wires are prone to failure. Stranded, you don't care much because even if you nick or cut a few strands, you still have more leftover. (Not so many more with mere 7 strand, something to think about; 19+ strand is much better in this regard.)
It may also be a pure matter of practicality: high current cables are best made from welding cable (lots of fine strands), or flat braid, easily manipulated by puny human hands. Else, bus bars and brackets, made from sheet or bar copper, are an opportunity, but the design needs to be very well considered as they are a complete, inflexible, mechanical component. You can build a cable harness in some minutes, but these things can take days or weeks of NRE. (I mean, simple point-to-point links you can cut and drill in minutes, but preparing drawings for actually purchasing parts, takes some engineering.)
Tim