They could have, but I think the goal for Stellantis was a global powertrain with standardized manufacturing for all their markets. Cost effective on their end, and as a European company they are primarily concerned with European emissions compliance. This engine will service both markets, and share parts. It’s in the name, global medium engine program.
V8 indeed can be done, but only really viable here in North America where we are performance based and not displacement taxed. Ford does it, the current 5.0 is running an extremely high 12:1 compression ratio with dual port and direct fuel injection. The 12:1 static compression can be done on pump fuel due to the 3,000 psi direct injectors they added to the V8 in gen 3. In lower rpm driving, the fueling is primarily port injection, as the Ti-VCT system retards intake valve opening to increase cylinder pressures with increased ram inertia, direct injection takes over as primary. At low rpm it runs 90/10 port-direct, and at high rpm it’s flipped at 10/90 port-direct. Tailpipe emissions on the 5.0 Ford are about the same as the 3.5 EcoBoost and this Hurricane, so a clean V8 is indeed doable. Stellantis isn’t primarily a North American player, so the idea of a new or evolving V8 program likely isn’t as compelling to them as it’s is to Ford/GM. Europe taxes based on cubic inches (an outdated law imo) but it is still their current rules.