Why do two words sometimes become one when the process has nothing to do with creating an adjective (and hence has some rationale at least)? There seems to be a tendency for the preposition 'in' to become permanently attached to a word that commonly follows it; to wit: 'infact', 'inturn', 'instore', 'inorder', to say nothing of the ubiquitous 'alot'.

I can see that we can confuse ourselves with many words that have both one word and two word forms, depending on meaning ('any one', 'any way', 'may be', 'can not', and so on), but what is the linguistic process or reason for joining words when there is no difference in meaning between the old and the 'new' form?