Well there's your problem. Valley pricing. You can get Sr Devs much cheaper than that. Then you can afford to train up some juniors.
I don't think that's the real problem though. Companies want you to take the risk/expense of learning a technology that might be obsolete next year. They'll pay you back in salary if you lucked up and mastered the right stack. If you didn't, then you keep eating ramen until you strike gold.
The other problem is most Sr.s don't want a Jr. coming in and making negative contributions. Sr.s are frequently the ones doing the interviews. They see most Jr.s as "What's a computer?" level talent that will start fires in production systems.