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Why are there so many packages being built

Because there are that many packages in the workspace.

It looks like the instructions linked are to build the core of ROS Dashing plus more from source, which is a lot of packages. Is that intended? If ROS Dashing is already installed then put only the packages you want to build (only the demo nodes you mentioned) in the workspace.

do these need to be built every time I make a new workspace

No. The workspace only needs to have the packages you want to work on. Put only the packages you want to build in the src folder of the workspace, then source /opt/ros/dashing/setup.bash before running colcon build.

is there a way to only build new packages if all of these need to be done in the future?

Colcon has a few options for picking which packages to build. Run colcon build -h and look for options starting with --packages-

--packages-ignore [PKG_NAME [PKG_NAME ...]]
--packages-ignore-regex [PATTERN [PATTERN ...]]
--packages-up-to [PKG_NAME [PKG_NAME ...]]
--packages-above [PKG_NAME [PKG_NAME ...]]
--packages-above-depth DEPTH [PKG_NAME ...]
--packages-select-by-dep [DEP_NAME [DEP_NAME ...]]
--packages-skip-by-dep [DEP_NAME [DEP_NAME ...]]
--packages-skip-up-to [PKG_NAME [PKG_NAME ...]]
--packages-select-build-failed
--packages-skip-build-finished
--packages-select-test-failures
--packages-skip-test-passed
--packages-select [PKG_NAME [PKG_NAME ...]]
--packages-skip [PKG_NAME [PKG_NAME ...]]
--packages-select-regex [PATTERN [PATTERN ...]]
--packages-skip-regex [PATTERN [PATTERN ...]]
--packages-start PKG_NAME
--packages-end PKG_NAME

You might be interested in the colcon build documentation.