The AGPL and GPL can be applied to output, since the output is a derivative of data licensed under the (A)GPL. Making a model with MB-Lab is not the same as writing a word document from scratch. The model is not conjured from the end-user's input. The sliders are illusions of user input and control; the programmer, not you, decided what happens when a slider is moved. To claim that a MB-Lab character model belongs to you because you moved a slider is similar to claiming that a Skyrim or Fire Emblem Awakening character model belongs to you because you moved a slider.
This is why exceptions to the AGPL and GPL exist. One fitting exception is Autoconf-exception-3.0:
AUTOCONF CONFIGURE SCRIPT EXCEPTION
Version 3.0, 18 August 2009
Copyright (C) 2009 Free Software Foundation, Inc. https://fsf.org/
Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.
This Exception is an additional permission under section 7 of the GNU General Public License, version 3 ("GPLv3"). It applies to a given file that bears a notice placed by the copyright holder of the file stating that the file is governed by GPLv3 along with this Exception.
The purpose of this Exception is to allow distribution of Autoconf's typical output under terms of the recipient's choice (including proprietary).
- Definitions
"Covered Code" is the source or object code of a version of Autoconf that is a covered work under this License.
"Normally Copied Code" for a version of Autoconf means all parts of its Covered Code which that version can copy from its code (i.e., not from its input file) into its minimally verbose, non-debugging and non-tracing output.
"Ineligible Code" is Covered Code that is not Normally Copied Code.
- Grant of Additional Permission.
You have permission to propagate output of Autoconf, even if such propagation would otherwise violate the terms of GPLv3. However, if by modifying Autoconf you cause any Ineligible Code of the version you received to become Normally Copied Code of your modified version, then you void this Exception for the resulting covered work. If you convey that resulting covered work, you must remove this Exception in accordance with the second paragraph of Section 7 of GPLv3.
- No Weakening of Autoconf Copyleft.
The availability of this Exception does not imply any general presumption that third-party software is unaffected by the copyleft requirements of the license of Autoconf.
This exception permits the end-user to own and choose the license of the output by making an explicit distinction between "Ineligible Code" and "Normally Copied Code" and spelling out what you may and may not do with each. However, without an explicit exception, such output would be automatically be covered by GPL. If it were otherwise, as you claim, then such exceptions wouldn't be necessary. To put it another way, the need to explicitly list an exception indicates that the rule and its associated problems do indeed exist, i.e. the exception proves the rule.
Another exception is the the MakeHuman's license exception: Unlike ML-Lab, they explicitly licensed their data under CC0 so that the end-user can choose the license for the output derived from that data. I have to wonder if the original question mistakenly conflated MakeHuman and MB-Lab together: MakeHuman uses a AGPL+CC0 combo, while MB-Lab uses a GPL+AGPL combo.