Why do high-church people call their personalized Bible study exercises "devotionals", as in the Catholic and high-church Anglican traditions?
The name 'devotionals' is actually fairly common, not just among high-church Christians. I've heard it used by Baptists, Mennonites, Anglicans of different persuasions and all sorts of people. There's nothing special about the name. "Our Daily Bread" describes itself as a 'daily devotional' and is certainly not 'high church'. The same applies to devotionals produced by crosstalk.com, biblegateway.org and Joyce Meyer.
However most important is that a 'devotional' is much more than just Bible study. 'Devotionals' may include prayer, worship, and possibly meditation, journaling or reflection. 'Devotionals' also applies to short homilies, which are again more than simple Bible study. Those that practice 'devotionals' will also do plain Bible study, which they call 'Bible study'. If it's more than that, they call it 'devotionals'.
It's an old word, not usually used as a noun these days. OED has
The use of what are now adjectives as nouns wasn't particularly uncommon in the seventeenth century.
It's a similar word to sacramental:
That is, just as a sacramental is analogous to a sacrament, a devotional is analogous to a devotion. Not actually a devotion, but an activity helpful to devotion.