Sex toys are not allowed because you are using a foreign object to pleasure yourself when only any part of the spouse's body can be used. In the same way lubrication such as KY Jelly is not natural but external to the human. Why don't the scholars argue that your natural lubrication from both of your privates is enough. Otherwise this is inconsistent with the sex toy ruling is it not?
Without confirming or denying the legality of this, the point on lube doesn't seem to be an equal comparison.
Necessity: Allah (ﷻ) designed male anatomy to penetrate females and provide pleasure. So the tools/toys are not even needed when men have their hands, mouths and privates [many different ways to sufficiently please the spouse]. On the other hand, medically speaking- not all women produce enough natural wetness to have comfortable sex and may be in pain otherwise. Cases like this would warrant the use of pure lubricants.
Competition/Substitution: Although both are foreign objects, the toys "compete" with male privates (and often are used for pleasure as a substitute by people who engage in homosexuality/lesbianism/masturbation -it's mostly used for evil) whereas the lubricant is an "aid" for the natural anatomy and by itself provides no pleasure [same story with condoms].