Why are the sperm donations hidden & the lies not shared till child is adult if at all?

Depends upon parents. Best to wait until child asks appropriate questions, but wait until they are old enough to understand. Certainly avoid lies. You can tell them that sperm entered mummy's tummy and fertilized one of her eggs. They don't have to told that sperm came from a donor. When older, or when child is an adult you can tell them that they weren't old enough to understand earlier, but, you didn't lie.
?shows anger/pain? It appears that health tap may be the wrong place to ask your question. Assisted fertility has long been an area where donations/use has been respected as privileged information shared at the will and discretion of the adults involved. If you have found out u or someone else in your life was the product of this process, u need to ask those involved.