Mainly most parents hold the belief that BTC, lacking government oversight and approval, is primarily associated with money laundering and illicit business activities.
Cash and bank credit have government oversight and approval yet are used for money laundering 100x more than Bitcoin.
Even custodial fiat services such as CashApp, Venmo, Zelle and Paypal are not so much better in terms of illicit usage. Also as private companies they have the right to seize you rmoney whenever they want. But at least there's an 18+ age limit earmarked on these platforms.