But are they more difficult?
To hack you need to spend time, and time is money. So hacking is worth it to those with very low incomes like students, or those that absolutely could never afford a more expensive unit, it's also done for fun. So the amount of people that can buy a 300$ 70Mhz scope that can be upgraded to 350MHz is larger than the amount of people that can buy a 2000$ 100MHz scope that can be upgraded to 500MHz. Of those, only a fraction would have the skills to reverse engineer the unit.
In my opinion more contributing factors are:
For higher end units, the much much lower market share compared to keysight/tek and therefore much less thinkerers picking them up in the second hand market.
For low end (cough!) units like rtb/rtc, i believe hobbyists actually buy them new, and since they cost 3/5 times the price of a hackable rigol/siglent the owners are far more fearfull of messing with them.