Honestly the real answer is plot convenience.
But the in-universe answer would be that legal practice is just not that hard anymore.
I don't mean this in the sense that all the laws have been simplified to the extent that expertise isn't required anymore (obviously this is not the case in light of all the differing legal systems of every planetary system you might encounter).
Instead, we are meant to believe that humans have advanced so much that a capable person (including, at a minimum, the senior officers) have advanced so much since our time, that they are completely capable of handling such cases themselves. Examples of advancement since our time include:
- people pick up fluency in languages routinely
- all 9-year-olds do calculus
- Picard is expert in whatever he puts his mind to, to dabble in Fermat's Last Theorem and the most advanced areas of archeology
- and, your examples themselves: any competent officer can practice the law in a meaningful way
In other words, humans have advanced so much that many fields simply don't require advanced expertise anymore.
