The prob­lem is that these sys­tems are sub­ject to par­a­sitic noise from their own com­po­nents, which slows down the process.
The vac­u­um of space is not real­ly a vac­u­um but is teem­ing with ran­dom quan­tum fluc­tu­a­tions as pairs of par­ti­cles and antipar­ti­cles are spon­ta­neous­ly cre­at­ed and then anni­hi­lat­ed when they col­lide with each oth­er. These process­es occur on extreme­ly short time scales and can been used to pro­duce ran­dom num­bers. The prob­lem is that these sys­tems are sub­ject to par­a­sitic noise from their own com­po­nents, which slows down the process.