Why did all packages ordered from the same place arrive separately?

Different delivery companies?
Sent at different times?

Held up by customs?

Well, on Amazon, for instance, there’s a place at checkout where you can request all items be sent ASAP (even if one takes a few more days than another), or have them sent in as few shipments as possible. I usually ask for everything in one box even if it takes longer, because I am not a Prime member and I don’t like to pay for shipping if I can help it.

Different warehouses and/or fulfillment houses.

As @Love_my_doggie says, different warehouses.

I worked for a large internet retailer. The wholesale companies where we bought products often shipped right to our customers. We never touched the items.

All sorts of reasons are possible. One could be they were out of one of the items, and so it was sent a few days later. Did you check the date on the postmark? Or, the items might have shipped from different distribution centers. It also could be the shipper didn’t keep all the packages together, but that’s less likely.