The process of getting a Permanent Residence Permit for the spouse of a US citizen is surprisingly difficult, lengthy, stressful and expensive (the process currently costs anywhere from $500 to $1610 - plus any medical expenses). The process is yet more difficult for families that are not financially well established.
Applicants must also meet the financial requirements based on form I-864P (which can be challenging for young applicants or those moving to the US but are not already employed there). If they cant, they must find family/friends who are willing to take on financial risk and ample administrative efforts on behalf of the applicants.
If we can make it possible and easy for illegal immigrants to live and work in the US, why not do the same for the spouses of US citizens?