They cannot be yet, before Jesus' death and rising, because Jesus through but His crucifixion and resurrection healed the human nature and transferred it from death to life, and it is absurd to think that prophets had some other shortcut to eternal life than this. Jesus simply speaks here proleptically that since the prophets did what was due and in their powers with relation to God who has chosen them as vessels of His message, therefore they will definitely participate in the future glory. Thus, even the prophets dreamed and coveted to see what the apostles see, but could not (Luke 10:24), and even if Moses had maximum possible glory available before the Incarnation of Logos, still even this amount of glory did not amount to Moses' participation in eternal life and eternal, unfading glory, for Moses' glory was transient and fading (2 Cor. 3:13), not increasing from glory to glory as, say, Paul's, and any Christian's glory (2 Cor. 3:18). But, of course, even prophets who deceased, will participate in the glory that Christ has given to humanity through His crucifixion and resurrection. That is why He spoke exactly about His pending crucifixion to Moses and Elias in front of His three chosen disciples on Mount Thabor that was to happen very soon in Jerusalem ("They spoke about his departure, which he was about to bring to fulfillment at Jerusalem" /Luke 9:31) ), and this was a fulfillment of great expectation of gaining new unfading glory in the Kingdom of Heaven for not only Moses and Elias, but for all the other prophets as well.
To give an analogy: there is an illness, say, in Burkina Faso, that is transmitted from father to son with an escalated damaging power; a doctor knows that if a patient, say, Igor, who has inherited this illness from his father, will keep a certain strict diet, abstain from alcohol etc. he will be able to check this illness, so that the next generation will be born without escalation, but, the doctor knows also that in some 20 years a medicine will reach Burkina Faso that will defeat the illness altogether. Now, Igor did all he could, obeyed all instructions of the doctor, thus Igor's son, Constantine is born ill, but not irrecoverably ill, so that he lives when medicine comes to Burkina Faso and he is healed altogether. Then, grateful Constantine takes flowers and wine, makes a libation on the grave of his father Igor and says: "My dear father, unless your fortitude and self-abstaining I would not be able to live before the arrival of the medicine!" Such are the prophets: they did their best before arrival of Jesus - the ultimate Healer, thus Jesus will definitely reward them with the Heavenly Kingdom, but only through and after His crucifixion and resurrection.