Biden lives in a huge family compound. It is highly likely that the reason his sons are successful lawyer/lobbyists is that they've been selling access to him and his acquaintances. There's no need for him to make or take the money directly.
That very well may be true. But, even without any ancillary income, Biden annual government salary should have given him much, much more than $150K net worth, even with the most conservative of investment strategies.
I spent 20 years as a USMC officer. When I retired, my net worth was WELL OVER $150K.