Well many, maybe even a majority of season tickets are similarly priced to (and sometimes less than) restricted availability off-peak tickets for the same journey. This thread is about travel from Retford To London (assume Kings Cross)
The cheapest off-peak return ticket is a CDR at £54.00
A weekly season from RET to KGX is priced at £23.63 each way (so £47.26 for Anytime travel)
Given that most season tickets are annual, then the fare would be £40.72, that's just 75% of a limited validity walk-up fare. Even allowing for the 1.5% interest the railway gets from an annual ticket, it's still a far better deal than some season holders try to make out.