Why don't they say it, Really? Explanations.
It's always been between him and her. Other people look at them and wonder what's going on between them. Other people wonder if they've realized, and whether or not they've confessed their feelings. They get asked about their relationship by people with less tact. Neither one of them have ever given a truthful answer. They either lie or do their best to avoid answering those questions altogether. Subconsciously they've only lied when the other wasn't present to hear the question or answer, and they avoid it when the other one is present.
It's not something that either one of them even really think about. Nobody else can possibly understand unless they've been through everything that Ichigo & Rukia have been through together. It started with the important act of piercing his heart with the blade of her soul. A connection had formed that neither could explain, nor did they try to. From there it grew into something huge and unexplainable.
When she was there for the revelations of how his mother had actually died, the connection they previously shared became a link. It was the first time in his life that Ichigo actually let someone completely in. Then came the way they made observations about one another. Ichigo noticably observed that she had surprisingly kind moments, and Rukia observed his growth rate and talent with quiet awe.
With all of this melding between two souls, a third party decided to throw a binding spell into the mix. That's right, in the heat of Ichigo's first battle ever with a menos grande, a powerful ex-captain put the two of them under a lasting binding spell. Even though Rukia was informed that she was in a binding spell and with him, she had no idea the extent of it. Now there was a complete fusion of souls, and new possibilities that both are unaware of are within reach for them. The spell didn't have the ability to affect their own feelings or their own hearts though, but the connection between the two is unbreakable outside of the death of one of the two.
When she was forced to leave him bleeding on the ground and go and face her crimes in Soul Society, Ichigo was prepared to face death without any regrets. If she wasn't around, his will to keep living faded considerably. Upon learning that the previously spoken of ex-captain had saved his life, his tone wasn't one of gratitude. He was sure he would never be happy again, never feel like he really wanted to be alive again...until he found out that he could save Rukia's life. That information relit the will and determination inside of his heart and soul.
Looking back, he was at his most powerful when he was fighting for her. He even went and trained to control his hollow against all odds because of her. He went lengths for her that he was afraid to attempt once he succeeded in saving her. (Like fighting and winning against Zaraki Kenpachi) When she rejoined him in the living world and made his heart race as she turned his frown upside down, something happened that had never happened before. Rukia snapped him out of his depression and actually admitted out loud to him that she keeps him in her heart. Ichigo smiled the biggest smile he'd smiled since he'd thanked her for making the rain stop, not that he was facing her for her to see it. That was probably as close to a confession that the proud girl would ever get.
When she broke the rules of Soul Society to help save their friend Inoue, she had smacked some sense back into him and reminded him that they're partners. All he could do is grin a little bit. Even though she's always the first one on his mind, he has other people he has to concentrate on whose lives are in danger. She seems to understand completely and never gets jealous or questions his intentions.
People always wonder if they know what they mean to each other and both of them inwardly roll their eyes in disgust. Nobody knows better than them how they both feel. And they don't need to say the words to understand it, actions speak louder than words anyways. Neither of them feel the need to embarrass themselves by speaking out loud what the other has already known for ages. They even both know in their hearts that they are soul mates. For the people who wonder and can't see with their own two eyes the love these two share, a deep pity should be felt. Ichigo and Rukia both know from the center of their souls to the bottom of their hearts that they love each other and that they're soul mates...meant to be together. For these two, for now, saying it would cheapen its value, since neither one is the mushy type. When one of them finally does say it out loud, the other may be shocked that the words were spoken, but will never be shocked by the feeling itself.
For these two confident people, stating the obvious isn't necessary since neither one of them have any insecurities when it comes to the other. They've both risked their lives for the other enough to not have any silly insecurities. That concludes the explanation for why they just don't say what they feel out loud.