When making a transgenic mouse, why can't you just mate 2 chimeric mice?

I read that when making a knockout mouse, you have to mate 1 chimeric mouse with a wildtype to generate a heterozygous mutant before mating that heterozygous mutant with another to generate the zygous mutant. So my question is why can't you just breed 2 chimeric mice from the start, why is it necessary to go through such a long process? Thank you!