In general special education is very expensive because of the intense labor costs. Some students require a 1 on 1 aide, special transportation, an extended school year, etc. All special ed students require a lower student to teacher ratio, specialized staff, evaluators, and people at admin to handle all of the required documentation.
However, it is legally required, which gives districts two choices. Pay up front and do it right (smart choice).
OR deal with even more expensive lawsuits, compensatory services, grievances being filed, possibly the state taking over your district/sped department, and the high staff turnover that goes with the above. Additionally, some of the kids with emotional/behavioral issues will turn your campuses into a shit show because their needs aren’t being appropriately addressed.
So, to answer your question, most of these districts don’t have the resources but have to find the money anyway. Grants can help for a time but don’t last forever. It’s a rob Peter to pay Paul situation.