I think it is usually the case that banks have legal restrictions on what they can invest depositor funds in, though? This varies by country, and can change over time based on what laws the current government feels like enacting or repealing, but separation between the banking/​loan-making and investing arms of financial institutions is standard in lots of places.
I have personally taken out a mortgage at ~1.6%, invested the money in a standard index fund, and made money, paying back the mortgage rather than renewing when the interest rate on offer was 6%. I imagine the same would be true of an investment loan, and know for a fact that investment loans are available, and the income tax code in my country makes their interest tax- deductible.
I think it is usually the case that banks have legal restrictions on what they can invest depositor funds in, though? This varies by country, and can change over time based on what laws the current government feels like enacting or repealing, but separation between the banking/​loan-making and investing arms of financial institutions is standard in lots of places.
I have personally taken out a mortgage at ~1.6%, invested the money in a standard index fund, and made money, paying back the mortgage rather than renewing when the interest rate on offer was 6%. I imagine the same would be true of an investment loan, and know for a fact that investment loans are available, and the income tax code in my country makes their interest tax- deductible.