Probably because you have installed another Magix product, possibly a demo, which has installed instruments. These can only be used in that program so are locked. To get them to work in Music Maker you can add them as one of the included options in the purchased program or, if you have used up all your options, you can buy them.
As per Graham, but mke sure that you are logged in to the Store - upper right, in the Media Pool. You should see your email address there. If you are not logged in, or did not log in after upgrading, your instruments will likely be locked.
And, just to add another piece of (hopefully useful) information to this, on top of what Graham and John said: It is very well possible that whatever locked instruments you're looking at did come from some sort of program or instrument demo installation. However, I would consider it almost more likely that they came from the installation of another full MAGIX music program you own (e.g. older Music Maker versions, Samplitude Music Studio, more recent ACID versions). Maybe you e.g. purchased the popular "Music Maker Premium + Samplitude Music Studio" bundle? If that is the case, you should enter the serial number for the product from which the instruments originated into the activation/license dialogue of the current Music Maker.
This activation mechanism will work for certain older Music Maker versions (from generation MX/18 to generation 2017) and for certain generations of Samplitude Music Studio. (Unfortunately, in the most recent versions of SMS, it will no longer give you access to all of the many instruments bundled with that product.) The license will be checked and bundled instruments will be gifted to your account as in-app store items, which in turn will make them available to you in Music Maker.