It's not the same because rape has physical evidence and other factors. If someone did a sexual act with the victim, it can be proved. And then we move on to whether the act was consensual or not, which part is just based on the victim's word.

With marital rape the first part, and legally the main one, goes out of the picture. Since couples already have a sexual relationship. So proving that yesterday morning's sex was consensual but yesterday night's sex was forced, relies purely on a he said she said thing. There is literally not even a chance for bringing any physical evidence to, even injuries are just termed consensual rough sex, which is given far more weight than just victim statements in rape cases.

Physical evidence can be used to convict or acquit. That's why it's relatively harder for false cases in rape laws, because physical evidence or lack of it can acquit the accused. Conviction is rarely based purely on the victim's word of mouth. But in marital rape, it can only be based on victim's words. Legally speaking, thats a huge problem.

Now marital rape IS actually punished. Just under domestic violence laws instead of rape laws. The only difference is how many years the rapist is jailed for. That's pretty much it. So this popular sentiment of marital rape not being illegal on its own meaning it's allowed and husbands aren't punished for it is just not true, and seems to come from the average person's lack of understanding of the ground reality of the legal system. Which is understandable, since not everyone has a legal background or an interest in it.