Sharing knowledge does not lessen your store, often it gets you more. Sharing plays a key role in relationships and bonding, happens in small steps and is assisted through community membership.
Sharing knowledge is comparable to sharing your lighted candle: you have the opportunity to share your flame with others who have unlit candles, increasing the visibility for everyone and losing nothing yourself or you may keep your candle to yourself and get by with a feeble flickering single candle which is eventually going to burn out anyway. By sharing the fire with others, we enable others to keep the fire going and grow it, benefiting everyone. Which scenario do you choose? Which scenario do you think most people choose? Why do you think people don't share knowledge more easily?
Sharing is more than access, it is helping others make meaning and obtain understanding. Deeper sharing is possible if the domain is partitioned, the core concepts are negotiated, the environment is scanned and the feedback is shared in community. There needs to be reflection around joint meaning and reciprocity to have quality sharing.