@JasminePrater
That is expected behavior with map image layer. That is because when you interact with map, one single request for a map image layer (no matter how many sub layers it contains) goes to the server. The map service draws all those layers for a single call on the server side and returns a resultant image that gets draped on the map in your web app.
Whereas for every single feature layer, web apps make at least one request... so, if you have 10 feature layers in your map, it will send 10 requests. And features with geometries come back in results and web map draw them on the client map.
That is why map image layers are remained in a group structure even though it might have only one layer in it.
If you don't want group structure, then as @Blevins_Mark suggested above, you need to add individual layer off a map service as feature layer.
you can also create feature layer portal item using similar url that Blevins mentioned above. And that will allow to browse and pick that feature layer. Again at that point it will not get added as a map image layer, rather a feature layer.