It's nonsense to say rehabilitation isn't possible. Organisations like Lucy Faithfull would strongly disagree.
I have read stats saying approx 1 in 100 men has accessed these images of child abuse, but that the true numbers are higher. There is no space in prisons to house that many offenders.
Also let's be clear we are talking about non contact where there is no perceived risk of contact. Generally there will also have been an early admission of guilt.
Those who commit this type of offence often have myriad problems (previous victims of childhood abuse, socially isolated, etc). Sending a person to prison is likely to exacerbate this. Treatment is a better option for those who have not committed contact offences, are assessed as not being at risk of doing so, and who admit their guilt and engage with rehab.
Most who commit this type of offence are very damaged individuals who want to stop but have no means of seeking help. Any HCP who is consulted would have to make a report to the police. So you can't simply go to a Dr or counsellor and ask for help without the very strong possibility of arrest.
The reality is given the numbers involved most police forces are struggling to keep up with investigations. Some (Sussex is one) are now giving offenders a chance to get treatment rather than going straight to seizing equipment. It's likely if this proves successful that other forces will follow.
Again this is purely for those who have viewed images. Not those who have shared as that's more serious, ditto the offender referred to upthread giving sweets to children.