I had this exact same problem with a Deadlands Reloaded campaign I am currently running. For my games I use a standard template for players to fill in their character backgrounds before the first session starts, which gives many opportunities for plot hooks to be identified.
However, one of my players despises doing this as part of character creation, so we agreed not to go through the process with her.
If you are unwilling to make changes to the plot in order to make their PCs more important then your options are limited to working with them by suggesting aspects of your planned plot arcs that might fit with their characters. Maybe you have an evil villain who it would make sense for one of them to have encountered before Perhaps they could have ties with a location that is going to be important to the plot further down the line.
Another approach is to tie their characters into plot hooks generated by those who do have detailed character backgrounds. One example of this I used is that one of the other characters has a 'taint' that acts as a beacon for certain types of monster. In order to tie this in to the backgroundless PC, I made it so that only she could see visible evidence of this taint. Doing this immediately created a connection - why could she see it and no one else? Where did she get this ability from etc? A second example is another PC whose parents were both killed by nasty people. Rather than going down the more obvious route of having the PC themselves haunted by their ghosts who wanted to pass on a message, I made it so that the ghosts chose to communicate through the backgroundless PC.
Finally, you can also take small details that emerge naturally through play that relate to the backgroundless PCs. There is a huge amount of flexibility here depending on what details come up. Perhaps there is an NPC their character seems to have a connection with. You could relatively easily increase the important of this person in some way to highlight the relationship with the PC.
In the end, if you're willing to be a little flexible with your planned campaign (and to be honest I view it as good practice for a GM to make changes based on what PCs are and do anyway), then you'll find it a lot easier. Changes really don't have to be significant in order to involve a PC more deeply. If you aren't prepared to go down this route though, I hope that my suggestions prove useful.