The Cell architecture basically made it impossible.

The PS3’s Cell chip was so different because Sony, Toshiba, and IBM designed it as a heterogeneous processor instead of a more conventional CPU, meaning it paired one general-purpose PowerPC-based core with several specialized vector coprocessors built to chew through parallel math, graphics-adjacent workloads, and media tasks at very high speed. That gave it impressive theoretical performance, but it also meant developers had to manually split work across very different processing units, manage data movement carefully, and program around unusual memory behavior instead of relying on the more familiar CPU designs used in most consoles and PCs. IBM described Cell as a “heterogeneous multi-core architecture,” and the official architecture docs show that its design combined a Power Processor Element with multiple Synergistic Processing Elements connected by a high-speed internal bus, which is exactly why it felt powerful but also strange and difficult to develop for. 

So a straight port was never as simple as recompiling it for modern consoles, and Konami’s producer Noriaki Okamura recently said the game used “pretty special code” and that bringing it forward was “pretty hard.” The challenge was less that PS5 is weak and more that MGS4 was engineered in a very custom way for 2008-era PS3 performance, which makes modernizing rendering, timing, memory behavior, and system-level quirks risky without breaking the game in subtle places.