Modern VRAM (GDDDR) runs at very high frequencies and very low latency. This makes the wiring between the RAM chips and GPU very tricky (they need to be short and the same length), so slots aren't an option. There were actually VRAM slots in some 90s GPUs.
VRAM runs at high frequencies, delivers insane throughput, but the latency is not great. GPUs don’t need low memory latency. They have very high degree of parallelism. GPU cores switch to other threads instead of waiting for data from memory.
