API: pd.concat with len(keys) != len(values) does not raise; intentional?
### 

- [X] I have checked that this issue has not already been reported.

- [X] I have confirmed this bug exists on the [latest version](https://pandas.pydata.org/docs/whatsnew/index.html) of pandas.

- [X] I have confirmed this bug exists on the master branch of pandas.


### Reproducible Example

```python
import pandas as pd
ser = pd.Series(range(5))
sers = [ser + n for n in range(4)]
keys = ["A", "B", "C"]

>>> pd.concat(sers, keys=keys, axis=1)
   A  B  C
0  0  1  2
1  1  2  3
2  2  3  4
3  3  4  5
4  4  5  6
```
```


### Issue Description

I expected the pd.concat below to raise.  It kind of makes sense as a reindex or analogue to the DataFrame constructor, but seems more likely to me to be a footgun.  In particular, I think the _lack_ of raising may be hiding a bug in the groupby code.

Is not-raising an intentional decision?  If so, thoughts on either deprecating or issuing a warning? 


### Expected Behavior

N/A

### Installed Versions

N/A