Assembling a crossover is pretty easy; place the components, solder them up, done. Designing a crossover takes a lot of work.
For example, building an engine is easy; organize parts, start putting things in place, torque to spec, etc... Put it together in a 1bay garage over a few weekends.
Designing an engine however, requires a team of engineers and thousands of hours of simulation and testing.
Off the shelf crossovers (generic crossovers), are just that, generic. Sure it's a starting point for speaker building, as in, it will make sound and protect the drivers, but it's a long ways away from creating a good sounding speaker. I'd hate for a person to spend $3-400 in raw drivers and another $50-60 in generic crossovers, to make noise when they could go off a proven design, or a kit, and spend an extra $30--40 in crossover components and have good sound. If the difference between mediocre and good was a mere 10% increase in price, it would be well worth it in my book!
Check into some DIY kits/designs that folks have spent time simulating, prototyping, testing, tweaking, repeating, etc... They're providing their time and knowledge free of charge basically.