The day (or month, or year) you want to send a probe to Pluto and the day you want to send one to Mercury, not to mention the direction, are going to be totally

The day (or month, or year) you want to send a probe to Pluto and the day you want to send one to Mercury, not to mention the direction, are going to be totally different, and aren't easy to schedule — you might have a 3-day window to be sure something can get to Saturn, since you're relying on a gravity assist from Mars and a configuration of the planets that only happens every couple decades. Also, even if such a thing could be organized, there's simply no way the budget will support 10 missions launching at once. They have to be tracked, developed, and funded separately for a lot of really good reasons. Stuff that's shared (launch vehicles, software) can be co-developed but each mission has wildly different timelines and requirements.