Because each custodian has an “adoption agreement” that contains restrictions that people and their advisors never read. A simple restriction may be that only publicly traded securities are permitted (stocks, bonds, mutual funds) but not privately traded securities or real estate. Other less obvious restrictions may be charges for transferring the account later or restrictions on your ability to set up your beneficiaries as you desire.
In fact, there are a number of restrictions, and you or your advisor should read the adoption agreement that your custodian provides before opening the account. (Note — any IRA-savvy advisor will have already done this and selected their “favorite” custodians that offer the flexibility you desire). Don’t be surprised if you ask your current advisor if he or she has read the custodian agreement and they say “Oh, that’s all boilerplate — no one reads it.” Change your advisor — fast.