Larger estates are complicated. You create trusts to avoid going through probate. There are often multiple trusts for different reasons. There may be tax advantages to creating trusts that don't count as part of the beneficiaries' estate because that would put them over the estate tax exemption. There are investment accounts that have to be distributed. There may be tax advantages to giving certain accounts to certain beneficiaries based on what state they live in. Houses that have to be sold. Tax laws change all the time so what may have made the most sense in 2019 might make less sense in 2021.

It goes on and on but the more money is involved and the more someone has done to avoid giving the money to the government the more complicated the distribution becomes.