Why do people think that rabbits and hares are different species?

Because they are in fact a different species.
There are 29 different species of rabbit so go figure.

Biological classification is the result of many people's years of hard work, not a matter of what "people assume."  The family Leporidae includes about a dozen rabbit genera and a single but very diverse genus of "hares."  Yes, you could call them "just a bigger sort of rabbit."  So what, biological nomenclature is the outcome of human decision-making.  The distinction between rabbits and hares is quite clear (go read about it), but if you had been in charge of classifying Leporidae, you might have made a different decision.  Tant pis.

In science it is OK to guess, but a guess needs to be backed up by research to have any value. You obviously do not know the first thing about rabbits and hares and are just talking out of your butt.

Why do many people think that they know better than experts who have spent many years studying a subject ?  Why do they ask stupid questions?

Rabbits and hares aren't rodents.
I recommend you read
Campbell Biology, 7th edition or newer
from cover to cover before you ask any more questions.

Firstly, they are not rodents but lagomorphs. Unlike rodents, they have a pair of tiny incisors behind the two big ones. And the ears are not always that large. Ear size correlates with temperature. Jackrabbits (actually hares, as are domestic "rabbits") have rather large ones, which dissipate excess heat. Snowshoe hares, which live in the Arctic, have shorter ears, which conserve body heat.
There are several differences between hares and rabbits. It's been more than half a century since my last course in mammalogy but I vaguely remember that they differ in whether they dig their own burrows and whether they are born altricial (needing care to survive) or precocious (ready to go).
With popular names so confused, it is no wonder that you are ignorant of the differences. No great harm in that, unless you post a question that insinuates that the people who know what they are talking about are wrong and you are right. Who do you think you are? Donald Trump?

> They're both rodents with big ears and fuzzy tails, they look basically identical, and they both have the same abilities.

And if ANY of that had anything at all to do with how we define "species", that would be a damned good point. But it doesn't, and you don't. Instead, you're an idiot.

People do not think that rabbits and hares are different species. They know thay are different species.

Hares and rabbits are not rodents.

Clearly you were bored and wrote this question randomly off the top of your head because you have made no attempt to verify your asinine claims.

Hares and rabbits do look differently and have different abilities.

Hares and rabbits, along with pikas, are lagomorphs - a different order from rodents.

Hares and rabbits are in the family Leporidae, making them leporids. I do not know why someone has told you there are 29 species. In fact, there are 61 species in 11 genera.