Because there is nothing special about Swift. His success this year is a product of the offensive line. He is easy to replace for pennies in free agency ( ~2M ) or day three of the draft.
His cap next year is 8.83 vs 1.33 dead cap. Take that 7.5 savings subtract around 2 mil for a replacement in FA and that's 5 mil for other things.
In addition to the obvious needs, the Bears also need at least one S and one LB. Look back to when they signed Edwards (LB), his first year cap hit was 5.3 Baird (S) was 6.4 his first year. Go find the next one of those.
This years NFL cap was 279.2 M. Currently Spotrac is showing Chicago at 300 M for next year already on the books.