If a biomarker dye can make cancer cells light up, why are we having such a hard time targeting them?

Assume by targeting you mean target them for treatment.

Then, human cancer don’t generally come with fluorescent dyes them self. Our cells don’t just glow red and green for fun.

So, problem one: how you put biomakers into your patients cells? You can try to use new tools like CRISP, but human trial is still in its infancy. Or you can try Lox P and other bunch of older tools, but inevitably you have to use some sort of trans infection to get those makers into human cells (cancer cells are human cells too), which is hard, and dangerous. Or you can use small compound markers, but how specific is your marker? Are they toxic?

Problem two: how you target those cancer cells? It’s not like shooting down a range, where you can see, you can shoot. Even you can light up cancer cells using dyes doesn’t guarantee you can point your drug to them. Also, deeply inbreed in the tissues, bio marker byes doesn’t help much. Presumably you have to cut open the covering tissue to see the abnormal color. Rather we use PET and NMR to indirectly see abnormal metabolism of cancers, which doesn’t need surgical procedures.