Brammo and Alta supposedly put a lot of effort into selling powertrains too. Zero sells powertrain components, but on a limited basis. It's usually best for small businesses to focus on their core product rather than burning engineering hours supporting customers who buy only parts, driving down gross sales and ultimately margin. If Energica wants to build drivetrains for smaller bikes they should build the bike as well rather than giving potential competition a leg up.
Source: Been there done that.
I'm no expert in this field, but have a lot of experience of scaling startups in the software business.
Energica itself is still startup-sized (<50 people, AFAIR), despite already being listed on a stock exchange. They don't have the marketing/sales/service/training organization that would be needed for large-scale expansion; while they do have a larger corporate parent, AFAICS that parent has no experience in automotive/moto consumer sales either.
Building such an organization takes a lot of money & time.
Under those circumstances, I can see the attraction of partnering with an existing large motorcycle-component supplier, who already has the relationships in place with moto manufacturers, and letting that supplier take care of sales & training.
Yes, you make less profit per bike that way than you would selling complete bikes, but it's also less risky, and I suspect Energica simply doesn't have enough money or time at this point to grow significantly otherwise. Scaling automotive manufacturing requires a lot of upfront investment, as does scaling the sales/service. Not every new moto company can do what Tesla did, relying on Silicon Valley to invest millions of VC capital.
It's notable that Zero isn't doing very well scaling their operations the usual way, slowly adding dealers -- the number of complaints about service is still quite large, and they've already pulled out of entire countries multiple times due to issues with distributors (and it doesn't matter whose fault it was in each case). OTOH, significant scaling is the only way they'll be able to lower the price of the bikes, which is a clear barrier for wider adoption (recall it took them a few years until 2015 when premium component makers Showa, Bosch & Pirelli agreed to supply them).
Also, Rimac have been doing a similar thing:
Selling EV components to various EV makers to fund the R&D of their own cars, and they now have a strategic agreement in place with Hyundai/Kia to help them develop high-performance BEVs, in exchange for an €80M investment.