Well they have dabbled in it since 2016 the with the CODE series which are all modellers ( typically 14 Marshall Preamps, 4 Marshall Power amps and 8 Marshall Cabinets) plus FX and computer connections for updating and additional presets.
They've also long held a small foothold in the digital market with the JMP-1 Preamp, the JFX-1 processor and even the more recent JMD-1 amp that is valved power but with an internal digital preamp and FX unit (
@Jethro Rocker has one in fact)
But with a company like Marshall, they have a simple recipe and it works: be one of the largest and most respected amp companies while focusing solely on that market. They do own a few subsidiary companies like Natal Drums and Eden Electronics, but their bread and butter is strictly amplifier manufacturing. It's what makes them unique, in that they WERE one of the last major amp builders that was family owned and wasn't owned by another instrument company ( that now would be Orange). Even Mesa Boogie became part of Gibson, so this is kinda big.