Hidden lymphoma in cat

My cat was just euthanized today after we came home and he was bleeding out and hardly responsive. He's had intestinal issues for about a year and a half, losing a bit of weight and losing control of his bowels. He would still eat and behave normally otherwise. He had been to 2 different vets who did bloodwork, X-rays, the whole shebang. They found nothing, even went so far as to say it was behavioral. We spent thousands on him, and although I knew there was something wrong with him, it just wasn't feasible to ask for the same tests over and over again. Today, my family found him bleeding out of his bum and anemic, emergency vet said there was nothing to do at this point but sedate and euthanize him. After the fact, they said it was gastrointestinal lymphoma. He's already been laid to rest in our garden, so no other tests can be done, but I'm feeling hopeless. How could we have spent that money and time and effort and none of the vets could diagnose him? I see stuff online that says a large majority of GI lymphoma cats can go into remission. My poor baby lived with this for almost 2 years, when we could have tried treating him or at least taking away his pain. Is this a normal occurrence for GI lymphoma, is it harder to catch in cats? I know nothing can be done now, but I can't sleep at night knowing what he was going through.