How a bacterial cell recognizes its own DNA

I want to know why the viruses haven't evolved to include Chi sites in their DNA.

My initial thought is perhaps a downwards pressure on the maximum length of the viral DNA, or perhaps Chi sites stop the DNA packaging properly.

tl;dr There's a sequence called a Chi sequence found very frequently throughout the bacterial genome, but rarely in the viral one.

Amazing the complexity of nature - I didn't realise bacteria had immune systems.