Why don’t modern gaming PCs use SLI anymore?

Having two physical slots doesn't necessarily mean a motherboard is ideal for multi-GPU use. Consumer CPUs and boards may not provide enough PCIe lanes for both cards to run at full bandwidth, and proper setups can require more server-like lane layouts. In many cases, buying one faster graphics card is simpler, cooler, and more reliable.

The biggest issue was frame pacing. Even when the average frame rate looked good, the two cards could deliver frames unevenly, causing noticeable stutter and jitter. Keeping both cards synchronized also created bandwidth and latency problems, especially as games started using much more VRAM.

Multi-GPU support depended heavily on both the graphics driver and the individual game. Developers often had to add and maintain a custom profile for each title, and most games never scaled close to twice as fast with two cards anyway. Since relatively few people used the feature, supporting it was difficult to justify.

Modern graphics APIs can technically support multiple GPUs through explicit multi-adapter features, but the game has to be designed for it. A few titles experimented with this, yet it never became common because the development effort is high and the audience is small. So the capability still exists, but there isn't much incentive to bring back a general-purpose SLI mode.