The wording of the spell prevents loopholes
Assume the wording of the spell said
You must use all of your movement to move away from the symbol on each turn
A clever player could then drop prone, and then crawl half as far as otherwise.
Or,
You must move your base speed away from the symbol on each turn
In that case, what about those whose movement is slowed by heavy armor? They are now the ones forced to dash. What about the dwarf in my campaign how has Boots of Striding and Springing? They will just have to move 25 feet, even though they could move 30 feet.
My point is that the wording of the rule may seem poor, but it actually prevents abuse by players and avoids unclear instructions in special situations. It says move 30 feet, so you move 30 feet.
It's harder for gnomes and halflings to fail this save
Gnome:
Gnome Cunning: You have advantage on all Intelligence, Wisdom, and
Charisma Saving Throws against magic.
Halfling:
Brave: You have advantage on Saving Throws against being Frightened.
Each has advantage to avoid the effects of this. If they fail, too bad for them. They have to struggle to keep up with the rest of the group, because the terror from the symbol is just as strong.
Dwarves, however, have no such ability (although lore-wise, most settings seem like they should have advantage against fear, but I digress.) So, I can see a valid complaint from dwarves.
So, a couple of the races with a 25 foot or less base movement speed already have means to help them resist the effect.
Powerful spells are powerful
Beyond that, Symbol is a very powerful spell, being 7th level. Looking at the effects of the other options the caster could use, it doesn't seem unreasonable at all to have this hard 30 feet movement, and so require some characters to have to Dash.
After all, if your hands are empty, you are already pretty much limited to screaming in terror and casting spells with only a verbal component. With not being able to keep anything in your hands, you aren't going to be able to do much with your action anyway. (Unless you want to litter the floor with all your equipment as you draw it use it once, before dropping it next turn.)