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Fair enough. But if "ten cubits" doesn't necessarily mean ten cubits, why does "six days" necessarily mean six days?

For me context is everything. And depending on context the two (cubits and days) can be apples and oranges. I'll try to explain what I mean.

Both cubit and day are linear measures of sorts. While we'll never have the exact measurement for a cubit because of the nature of the measurement, we can calculate an exact measurement for a day. Yet through-out the Bible a day can mean different measures of linear time depending on context.

Here is some information for interpreting "day" in the Bible

The singular word "day" is not known to be used in the OT for a long period of time. This is worth noting as there are over 2200 references to "day" in the OT.
The singular "day" is used for
- a twenty-four hour period
- the time of daylight
We see this in Genesis 1:5. It can also be used indefinitely for a point in time, particularly in the future (such as in the phrase "day of the Lord"). It's use in Genesis 2:4 ("in the day that") can mean
- when
- after
- at the time of
And carries no special meaning. It's used as an analogy in Psalm 90:4 where one (or part of a) day is like "a watch in the night."
Time references to God in the OT are used in the normal sense. The reference to God's "years" in Psalm 102:24,27 really only makes sense if the years are understood in normal usage.
In reference to a period of time, the plural form "days" is used. We see this in Genesis 26:1.
When the word "day" is used with a specific number, it always references a normal day. In addition to references in Genesis chapter 1, compare with Genesis 8:14. There appears to be one exception, Zechariah 14:7.
- the usual Hebrew word for "one" is not used as a number
- the word "day" refers to daylight, not the full day or any other definite or indefinite period of time
Where the phrase sometimes translated "one day" does not function in the normal way and would be better translated "only daylight". Zechariah 14:7 is a reference that there will only be daylight in the new heaven and earth. If the phrase were used like this in Genesis 1, then evening and morning would be inappropriate, as would references to subsequent days.
In the entire Old Testament, when the word "day" is used in a numbered series, it always has reference to a normal day as through-out Numbers chapter 29
The reference to "evening and morning" for each of the six days of creation entails the normal daily interchange of light and darkness. If "day" is not understood in it's normal sense, then "evening" and "morning" cannot be either. This is never the case in the OT, even when cited in Psalm 90:5-6 where they are used in an analogy.
The reference to the days of creation in Exodus 20:11 and Exodus 31:17 in connection with the Sabbath law makes much more sense if understood in terms of a normal seven day week. Also, the above two references to creation are not used as analogies. It is stated as an imitation of God or a divine precedent to be followed - God worked for six days and rested on the seventh, so should you.
Evening and morning are not mentioned for the seventh day. Does that make the seventh day an indeterminate period of time? Genesis 2:3 seems to say no, where God blesses and hallows that day, making it clear it's a specific special holy day. Then in Exodus 20:11 that blessed and hallowed day is identified with the normal Sabbath day.
Some of Hebrews chapter 4 hits on the topic. Look at verse 4. But this text does not address the length of the seventh day of creation.

I believe the only way to interpret "day" to mean anything other than what the context appears to say in context is to be guilty of eisegesis. A rule of proper hermeneutics is to use the Bible to interpret itself, and when done correctly, the context should tell you the original intent.

Does that answer your question?