Why do bullets have steel cores?

Many munitions feature a steel core in their bullets construction, and many attribute the main reason for that being armor penetration, However thats not always the case. Also while on the topic why not go into bimetal jackets?

[@itsjustjoe3790]

Excellent video! You used wonderful visual aides. This will hopefully be a million view video! Thanks for the graphic presentation. Some day I’ll ship you those blue plastic 7.62 rounds and you can give us more data!