YABU. The punishment is loss of their freedom, home, job and professional standing, relationship, money, social life, hobbies (mostly), social life, access to their belongings, ability to see their kids, friends and family except for short visits.
Removing their few sources of mental stimulation and exercise would just cause them to deteriorate while they serve their sentence. They can hardly walk the dog or go for a round of golf.
In many countries including the UK, one of the key aims is to rehabilitate, not just punish. Counselling, therapy , education, training and jobs are important. You can’t achieve that by depriving them of everything. It’s notable that in countries where prison is seen as “lenient” because conditions are humane, and the emphasis is on rehabilitation, the rate of reoffending is lowest. Harsh conditions, dehumanising treatment and deprivation just don’t work. At the end of their sentence the prisoners go back into the world full of anger and bitterness, they’re institutionalised and unable to live normally with other people and re-enter the world of work. This is seen very clearly in countries where prison is seen as retribution and a “tough” experience - reoffending rates are high and prisons turn out an underclass of unemployable people who have become embedded into the world of criminals.