Why doesn't more memory necessarily make Postgres more performant?
In fact, effective_cache_size coupled with shared_buffers could more appropriately be considered to be the quintessential memory settings. Keeping shared_buffers a bit lower (e.g., 25%) is useful because Postgres also relies on operating system caches as well, which may account for utilization of some of the other "6GB" of RAM in the OP.

According to the official Postgres "tuning" page, setting effective_cache_size to half of total memory is considered to be conservative. However, this isn't a memory allocation, rather a guideline to help Postgres plan its queries based on what's available to it.

Also note that understating resources to Postgres by a slight degree can be helpful, in allowing some breathing room for future scaling. Imagine your Postgres server was optimized to take 100% advantage of all the physical resources in your machine, and then you reached your server's limit. There would be little you could do at this point to stave off disaster (e.g., swapping, extreme performance degradation, etc.), so leaving a bit of wiggle room can come in handy when you need a week's time to upgrade your server.