If they aren't voters that is something that is a problem that needs to be fixed. Illegal migrants make up something like 3% of the US population. It isn't fair for 3% of the resident population to be denied a right to vote; it'll become a serious social issue.
How would you like it if I break into your house and start living with you against your will, and then boss you around deciding what we eat for dinner, how our budget (your salary) should be spent, and so on?
If I'm not going to kick you out, you would get a say in dinner, the budget and so on. What alternative can you see? You're in the house. We live together. Best to get along.
Either the US goes genocidal and purge 3% of their population or they all get along. Getting along seems like the good option to me!
1. By having children that end up voting for the party advocating welfare/free healthcare for illegal immigrants, citizenship for illegal immigrants, open borders...
2. By living in states like California that did that Machiavellian 1-2 combo of allowing illegal immigrants to get drivers licenses, then made it so that people with a driver's license are automatically registered to vote.