Serialized defaults#

The value a serialized variable is set to in code is only the default when a new instance is created from scratch.
After the value has been serialized, that value will override anything that it's initialised to code.

Resolution#

I want to keep the value serialized so I can modify it in the Inspector#

Set the value to the variable via the Inspector.
The value in code is only a default for new instances. You can also right-click on a component header and Reset it to its default values.

I want the value to only be set in code#

Either make the variable private or protected, or mark it with the [NonSerialized] attribute so it is not serialized by Unity.
Generally it's a good idea to keep parameters serialized so you can tweak the values without editing code and recompiling.