So a couple of things but overall the main one. Goofy and Donald are after all. DISNEY characters. So one... copy right... if disney says no to them being keyblade weilders then disney says no. and disney only approved of Mickey cause he is the king. HOWEVER. Even if disney said yes, Key blade weilders are specific people chosen to play roles, as guardians. Goofy and Donald, have no interest in THIS SET ROLE. their goal is simple, help Sora and above all else, the KING. The only reason they joined Sora in the first place is because mickey told them to. Furthermore, Mickey has a keyblade cause as the king it is his responsibility to protect his kingdom thus he holds the role as guardian. Donald and Goofy have no real reason to hold that role cause at the end of the day, their prime job and goal is to just protect the king. At which point they have their own set of weapons designed for just that, wand and sheild. Both their weapons are symbolic of their goals and what they hold special in their hearts. No disney character aside from the king really has any reason to uptake the goal to become a guardian.
EVEN FURTHER MORE. If you remember, Kairi Riku and Sora have very set reasons for getting their keyblades, Remember Ven Aqua and Terra? The keyblade must be passed down. Ven passed his to Sora, Terra his to Riku, Aqua's to Kairi. No one has passed them down to Goofy or Donald and honestly none of them have reason to. The king.. sure he could... he'd pick one.... which one would become a ugly debate... and on top of that why would he chose one over the other? further more, again, he has no real reason to as that is not Goofy and Donalds goal or purpose. Their interest is to protect the king.
As for Riku Kairi Lea becoming keyblade weilders...no....Riku makes complete sense lol. Why would Riku NOT have a keyblade? Did you forget Riku was the main candidate for the keyblade? Sora got the main Keyblade only because Riku fell to darkness, otherwise Riku would have been the major player not Sora and Sora would have been the side character. I feel you saying Riku shouldnt have a keyblade is like... forgetting the whole first game and all its addon's KH1-KH1.8.
As for Kairi... No... Kairi makes sense. specially with where the game is heading.. one she was passed it down from Aqua, two she is a princess of heart and thus qualifying her more then most, to become a guardian.. honestly... the crappy part of Kairi... is that they didn't give her one sooner... As she remained way to weak way to long. Thankfully they fixed that and are now from the looks of it moving towards a SRK format in the future games. (much to my hope. as ive said else where and will say again, the series has a LOT to benefit from moving to SRK over SDG.) Kairi needed a keyblade and hopefully will continue to grow more as a character in future games.
As for Lea... he kinda became a part fo the main crew, you could debate his keyblade though tbh. As a member of the orginization he was worthy of one and strong enough for one.. but honestly... I think it's just cause hes such a cool character that holds such a large fanbase that they gave him one. as honestly, there isnt a lot backing him up for one tbhh, other then they needed another guardian of light. Not that im actually against him getting one. he is an important part of the team, and plus with him being so closely tied to Roxas and Xion both who are keyblade weilders, it does fit for him to have one so he can better assist his friends.