[twic]

> the algorithms are well known and only need to be implemented once

To an extent. The basic algorithms are well known (simplex, interior point), but there is a lot of scope for improvements - this is why the big commercial solvers can be orders of magnitude faster than the best open source ones. Still, even if the algorithms are not well-known, they do only need to be implemented once.

For integer programming, though, there can definitely be value in problem-specific heuristics for branch selection and rounding.