What makes you say that?
Either private insurance would cover it, or Medicaid (for low income people) or they essentially give it away to uninsured people through a non-profit foundation. (AbbVie Patient Assistance Foundation)
Certainly, if you were to flick through various forums for those with Crohn's disease (e.g. [1]), you'd find many stories of people paying hundreds (or even thousands) of dollars for a month's Humira treatment.
[1]: https://www.healingwell.com/community/default.aspx?f=38&m=22...
That's for the lowest benefit, qualifying ACA plans and assuming the prescription is covered. The "cash" price is ~$4000 a month (https://www.goodrx.com/humira ) so even the lowest benefit plans are kicking in quite a bit of support (but probably also negotiating the price down quite a bit).